Geo: Produce create events for new Package files

Problem to solve

• Produce create events for new Package files.
• Feature flag replication of Package files.
• While implementing the Self-Service Geo Framework. Take the time to make it reusable.
• [-] Write the Self-Service Geo documentation (See Documentation section below)
• Ensure this is well tested, generically. When adding a new Blob data type, we should be able to add a shared example to the model spec that checks that the create event happened.

In the next issue, we will consume these create events.

Intended users

• Devon (DevOps Engineer)
• Sidney (Systems Administrator)

Further details

Copy at-will from !20672 (closed) and !21486 (closed), and consider the discussions.

• The organization of event code in the POC is not necessarily finalized. Please open discussions about it on the WIP.
• Decide whether events will piggy-back on geo_event_log or not.

Proposal

Documentation

This is the external interface to the users of the Self-Service Geo Framework (non-Geo developers), so it's extremely important, and should inform our design as we go.

• [-] Create something like doc/development/geo/replicate_a_new_data_type.md
• [-] Link to it from https://gitlab.com/gitlab-org/gitlab/blob/v12.6.3-ee/doc/development/geo.md#steps-needed-to-replicate-a-new-data-type
• [-] Add caveat about it being alpha or under development
• [-] In the new doc write a tentative outline for what a non-Geo developer would need to "Replicate the creation of new files for new blob type X"
• [-] Write the specific content that a non-Geo developer would need to accomplish "Produce create events for new blob type X"

This was done in !23260 (merged)

Testing

What does success look like, and how can we measure that?

What is the type of buyer?

• Premium
• Ultimate

Links / references