[chore] remove type switch in Create() and instead move to FederatedCallbacks()

[NyaaaWhatsUpDoc]

Description

This is once again another part to #1891.

It moves the type switch out of the federatingDB{}.Create() function and instead handles them as separate functions that we pass in FederatedCallbacks(). Also has some other minor refactoring (no logic changes), and adds some new gtserror functions that should collapse some of our common error wrapping logic down to one-liners.

Checklist

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[tsmethurst]

Oh great stuff :) Been meaning to do this for tiiiiiime