Add Attachment Support for Mail Executor

[yottahmd]

Currently, the mail executor sends plain text emails without file attachments. We should extend its functionality to include an attachments field in the configuration, allowing users to specify one or more file paths to attach to outgoing emails.

How to Implement:

1. Extend the Configuration Schema:
   Add an Attachments field (a slice of strings) to the mail executor’s configuration structure. For example:

   type MailExecutorConfig struct {
       To          string
       From        string
       Subject     string
       Message     string
       Attachments []string
   }

2. Update the Mail Sending Logic:
   Modify the mail executor code that builds and sends the email to loop through the Attachments list and attach each file. Be sure to include file existence checks and error handling.

3. Update Documentation and Examples:
   Add an example in the documentation to show how to use the new field in a DAG file.

Example Usage in a DAG File:

steps:
  - name: send report
    executor:
      type: mail
      config:
        to: [email protected]
        from: [email protected]
        subject: "Daily Report"
        message: "Attached is the daily report."
        attachments:
          - /tmp/report.csv
          - /tmp/summary.png

[ghansham]

It can have fields for mail server configuration like smtp or tls with port configuration. What about passwords for user accounts? How to store them?

[yottahmd]

Good idea! Currently Dagu's stmp setting is defined at the DAG level. It is possible to add a separate step-level field, allowing user to specify different settings for each steps. I'd be happy to add that if anyone submits a PR.
Regarding secret values, we currently support environment variables via dotenv file. Env vars are not logged or stored in persistent files. In the future, I plan to add support for secret management services such as AWS SecretManager, Hachicorp Vault, and others.

[ghansham]

Yeah I saw in the reference documentation for dag level settings. We could add support for TLS in dag level settings.