[chore/security] refactor AuthenticateFederatedRequest() to handle account deref + suspension checks

[NyaaaWhatsUpDoc]

Description

Refactors our very crucial .AuthenticateFederatedRequest() function to handle new instance / account dereferencing, along with suspension checks. This reduces the need for these checks (and possibility of them being missed) in other areas of the codebase.

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 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]

Looks good to me, just a couple comments :)