Adding SMTPKit.com Template (#579)

Add smtpkit.com template

# Description

Aiming to enhance our domain validation services, the SMTPKit team is
submitting this template to streamline integration with Domain Connect.
This addition ensures a smoother and more standardized implementation,
improving compatibility with the platform’s services.

We appreciate your review and are available for any necessary
adjustments.

## Type of change

Please mark options that are relevant.

- [X] New template
- [ ] Bug fix (non-breaking change which fixes an issue in the template)
- [ ] New feature (non-breaking change which adds functionality to the
template)
- [ ] Breaking change (fix or feature that would cause existing template
behavior to be not backward compatible)

# How Has This Been Tested?

Please mark the following checks done
- [ ] Schema validated using JSON Schema
[template.schema](./template.schema)
- [X] Template functionality checked using [Online
Editor](https://domainconnect.paulonet.eu/dc/free/templateedit)
- [ ] Template is checked using [template
linter](https://github.com/Domain-Connect/dc-template-linter)
- [ ] Template file name follows the pattern
`<providerId>.<serviceId>.json`

# Example variable values
<-- to make review process easier please provide example set of variable
values for this template -->

<-- Example: -->

```
smtpkit_code: smtpkit-code:4e22f31e3032e7e7d86df7cabc1b8e24
dkim_selector: mail
dkim_public_key: v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC7vJ/Dgoaedp367Fc2xSwzq0liCrVTmLtGGa31D9V0NGu2usNFI+7sGyQQuSjfAsZvFJEdIHIDjA352hKFNuSwRL/517Q114x//alUCwvOx9n//b12N1Y0KGEpBvIDLWGgd1kpcN80Cowx0RLCxOB6KnUxsD0PBNknE5Uxjv9B1QIDAQAB
dmarc_value: v=DMARC1; p=quarantine; pct=100
```

<-- Or provide the whole `testData` object from the [Online
Editor](https://domainconnect.paulonet.eu/dc/free/templateedit) after
testing and using "Add as test" button -->
```
 "testData": {
    "Test data": {
      "variables": {
        "domain": "2jdev.com",
        "smtpkit_code": "smtpkit-code:4e22f31e3032e7e7d86df7cabc1b8e24",
        "dkim_selector": "mail",
        "dkim_public_key": "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC7vJ/Dgoaedp367Fc2xSwzq0liCrVTmLtGGa31D9V0NGu2usNFI+7sGyQQuSjfAsZvFJEdIHIDjA352hKFNuSwRL/517Q114x//alUCwvOx9n//b12N1Y0KGEpBvIDLWGgd1kpcN80Cowx0RLCxOB6KnUxsD0PBNknE5Uxjv9B1QIDAQAB",
        "dmarc_value": "v=DMARC1; p=quarantine; pct=100"
      },
      "results": [
        {
          "type": "TXT",
          "name": "@",
          "ttl": 3600,
          "data": "\"smtpkit-code:4e22f31e3032e7e7d86df7cabc1b8e24\""
        },
        {
          "type": "TXT",
          "name": "@",
          "ttl": 6000,
          "data": "\"v=spf1 include:smtpkit.com ~all\""
        },
        {
          "type": "TXT",
          "name": "mail._domainkey",
          "ttl": 3600,
          "data": "\"v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC7vJ/Dgoaedp367Fc2xSwzq0liCrVTmLtGGa31D9V0NGu2usNFI+7sGyQQuSjfAsZvFJEdIHIDjA352hKFNuSwRL/517Q114x//alUCwvOx9n//b12N1Y0KGEpBvIDLWGgd1kpcN80Cowx0RLCxOB6KnUxsD0PBNknE5Uxjv9B1QIDAQAB\""
        },
        {
          "type": "TXT",
          "name": "_dmarc",
          "ttl": 3600,
          "data": "\"v=DMARC1; p=quarantine; pct=100\""
        }
      ]
    }
  }
```

smtpkit.com.domain-auth.json

@@ -0,0 +1,44 @@
+{
+  "providerId": "smtpkit.com",
+  "providerName": "SMTPKit",
+  "serviceId": "domain-auth",
+  "serviceName": "SMTPKit Domain Authentication",
+  "version": 1,
+  "syncPubKeyDomain": "smtpkit.com",
+  "syncRedirectDomain": "sync.smtpkit.com",
+  "logoUrl": "https://smtpkit.com/assets/media/logo/android-icon-192x192.png",
+  "description": "Places TXT record for domain verification and DKIM records to authenticate email sent by SMTPKit on behalf of the user",
+  "variableDescription": "The variable %smtpkit_code% represents a combination of prefix string and a MD5 hash.  %dkim_selector% and %dkim_selector2% is the DKIM selector for placing the DKIM public key. This needs to be a variable because the selector is dynamically generated. %dkim_public_key% is the DKIM public key. %dmarc_value% is dmarc value.",
+  "records": [
+    {
+      "groupId": "smtpkit_code",
+      "type": "TXT",
+      "host": "@",
+      "data": "%smtpkit_code%",
+      "ttl": 3600
+    },
+    {
+      "groupId": "spf",
+      "type": "SPFM",
+      "spfRules": "include:smtpkit.com",
+      "host": "@",
+      "ttl": 3600
+    },
+    {
+      "groupId": "dkim_raw",
+      "type": "TXT",
+      "host": "%dkim_selector%._domainkey",
+      "data": "%dkim_public_key%",
+      "ttl": 3600
+    },
+    {
+      "groupId": "dmarc",
+      "type": "TXT",
+      "host": "_dmarc",
+      "data": "%dmarc_value%",
+      "ttl": 3600,
+      "txtConflictMatchingMode": "All",
+      "essential": "OnApply"
+    }
+  ]
+}