Specify email templates as separate configuration keys

[aeneasr]

Preflight checklist

• I could not find a solution in the existing issues, docs, nor discussions.
• I agree to follow this project's Code of Conduct.
• I have read and am following this repository's Contribution Guidelines.
• This issue affects my Ory Cloud project.
• I have joined the Ory Community Slack.
• I am signed up to the Ory Security Patch Newsletter.

Describe your problem

Currently, you can override the whole directory of where email templates are mounted from using:

courier:
  template_override_path: /my/path

The problem with this is that we cannot use any remote locations, and it's difficult to specify precedence.

Describe your ideal solution

Instead, it should be possible to specify for each template the location:

courier:
  templates:
    recovery:
      invalid:
        email:
          plaintext: # base64://, https://, file://, ...
          html: # base64://, https://, file://, ...
        # In the future maybe:
        # sms: ...
    # recovery ...

Workarounds or alternatives

None so far, if you have better ideas please LMK :)

Version

master

Additional Context

• Write migration guide
• Write breaking changes
• Document configuration with a guide / examples
• Document how to do translations ( https://github.com/ory-corp/cloud/issues/1695 )