[feature] Add /api/v1/admin/debug/apurl endpoint

[tsmethurst]

Description

If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.

If this is a documentation change, please briefly describe what you've changed and why.

This pull request implements a new api endpoint: /api/v1/admin/debug/apurl.

The new endpoint is built and included only if GtS was built with DEBUG=1 and run with DEBUG=1.

The endpoint allows admins to make GET requests to arbitrary ActivityPub urls via the running GoToSocial instance, and returns request and response headers, response body, and response status code.

This should help a lot with debugging odd ActivityPub issues with instances that enforce authorized fetch.

The outgoing GET request is signed using the key of whichever admin user made the API request, so it will still respect domain blocks and user blocks from the remote instance to the GtS instance, by relying on the remote instance to enforce ACLs.

Still a draft because I want to ensure I've got the build tags correct

Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: [ ] -> [x]

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

• 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.