ServiceDesk: Add `external_author` alias for `service_desk_reply_to`

On the Issue model, since service_desk_reply_to is used for both cases:

• Which external email addresses are receiving updates to this issue?
• Which email address originally created this issue via Service Desk?

Let's semantically separate these things by renaming the column on issue from service_desk_reply_to to external_author.

As a first iteration, the email address in external_author should receive notifications as if it was service_desk_reply_to. Copying of the external author to the external participants will take place as part of #226988 (closed).

No feature flag needed since functionality does not externally change.

(note that this issue description has changed so the discussion below is for the original context, which was "add single email participant via quick action")

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