Entity Mailer: common Mailer for business; common message format; track messages for idempotent processing

[thorwolpert]

Entity Mailer should be used for all business ops (current entity & namex)

Entity Mailer should move to using cloud event messages (currently namex)

Entity Mailer should track incoming messages and ones completed so that they get processed only 1x to completion.

Entity Mailer should propagate the CTX values and identifier in cloud-event to other systems, logs and history storage.

At least part of the work for this ticket is being covered in this bug fix: https://app.zenhub.com/workspaces/entity-5bf2f2164b5806bc2bf60531/issues/bcgov/entity/7882

[jdyck-fw]

This is likely about a day or two worth of work.