[SG-656] Update API endpoint to use RegisterResponseModel

[addisonbeck]

Type of change

- [x] Bug fix
- [ ] New feature development
- [ ] Tech debt (refactoring, code cleanup, dependency upgrades, etc)
- [ ] Build/deploy pipeline (DevOps)
- [ ] Other

Objective

On #2278 I updated the Identity register endpoint to return a model, but I did not update the API register endpoint. This was because I was concerned they changes wouldn't be backwards compatible with old clients.

Turns out there is a bigger reason to update both register endpoints: those endpoints are still used by all self hosted servers. To keep self hosted registrations working I've updated the API register endpoint with the new changes. The changes are still backwards compatible with old clients because old clients will have marked the register endpoint as not having a response and will not try to parse one.

Code changes

• Move the RegisterResponseModel classes to Core to be used by API and Identity
• Copied the Identity register endpoint changes to the API register endpoint
• Append the comments in this area to mention that self hosted installs still don't use the Identity endpoint
• Add necessary dependencies to tests

Before you submit

• Please check for formatting errors (dotnet format --verify-no-changes) (required)
• If making database changes - make sure you also update Entity Framework queries and/or migrations
• Please add unit tests where it makes sense to do so (encouraged but not required)
• If this change requires a documentation update - notify the documentation team
• If this change has particular deployment requirements - notify the DevOps team

[differsthecat]

TIL 😯

[Hinton]

It's actually used for all clients unless they run in dev since we needed to maintain backwards compatibility. I've created a PR to resolve this though since sufficient time has elapsed.