Add support for MX records in Azure, GCP and AWS

[Sewci0]

Description

Add support for MX record types in Azure, GCP and AWS. This still needs to be explicitly enabled by specifying a CLI flag --managed-record-types=MX.

Tested on all 3 cloud providers with a CRD source:

apiVersion: externaldns.k8s.io/v1alpha1
kind: DNSEndpoint
metadata:
  name: test-mxrecord
spec:
  endpoints:
  - dnsName: example.com
    recordTTL: 180
    recordType: MX
    targets:
    - 10 mail.example.com
    - 30 mail.foo.com

Checklist

• Unit tests updated
• End user documentation updated

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: Sewci0 (25058df)

[odinuge]

/ok-to-test

[szuecs]

Please add:

• a test for aws
• end user documentation
• document why nolint directive was used (it's fine but for our future self we should make sure we remember it

[Sewci0]

@szuecs thanks for looking into this! I've pushed all the changes that you requested. Please let me know if there's anything missing.

[szuecs]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Sewci0, szuecs

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• OWNERS [szuecs]

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[Sewci0]

@szuecs PR seems to be missing the lgtm label. Should I ping someone to get that added? Thanks!

[szuecs]

I thought @odinuge would maybe add the lgtm

[szuecs]

/lgtm