Option to add "X-Mailer" header when sending emails

This issue has been migrated from [#13182](https://github.com/matrix-org/synapse/issues/13182).

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The X-Mailer header is commonly used to identify the software/SMTP-client used to initiate and send an email.
For example, all emails sent from Facebooks servers (Instagram, Facebook, Whatsapp) have the following X-Mailer header:
`X-Mailer: ZuckMail [version 1.00]`

Example of different X-Mailer headers in my SMTP-relay

Why?
To make it easier for recievers to build filters depending on the X-Mailer header, and to just keep track on what kind of server the emails originate from.

Example header:

`X-Mailer: Matrix-Synapse ${CURRENT_VERSION}`
Or, the option to specify in the settings.