Add API endpoint to request contents of multiple files simultaniously

[denyskon]

Adds an API POST endpoint under /repos/{owner}/{repo}/file-contents which receives a list of paths and returns a list of the contents of these files.

This API endpoint will be helpful for applications like headless CMS (reference: sveltia/sveltia-cms#198) which need to retrieve a large number of files by reducing the amount of needed API calls.

Close #33495

[wxiaoguang]

Some thoughts in my mind:

1. Why it is a "POST" endpoint? If I understand correctly, it "GETs"
2. Does GitHub have a simliar API? If yes, we'd better to follow, if no, let's invent
3. It needs some tests (almost all existing APIs are covered by tests, otherwise it would easily break in the future if something changes)

[denyskon]

1. IIRC, the HTTP spec says or has said that GET requests should not contain a body - that's why it isn't an option. Post is the next best choice I think.

2. GitHub doesn't have a similar API, as such functionality is now being served through their GraphQL API. This, however, is clearly out of scope for near future, so I think that such an API endpoint would be helpful :)

3. I can add some tests...

[silverwind]

1. IIRC, the HTTP spec says or has said that GET requests should not contain a body - that's why it isn't an option. Post is the next best choice I think.

HTTP QUERY is a proposed standard to solve the GET-with-body problem. Tooling support is probably not there yet, but if it works in your case, it might be an option to use it already.

[wxiaoguang]

1. IIRC, the HTTP spec says or has said that GET requests should not contain a body - that's why it isn't an option. Post is the next best choice I think.

I see, I missed the Files []string field, it would be quite large so it is POST's job.

[denyskon]

@wxiaoguang Added an integration test

[lunny]

Except https://github.com/go-gitea/gitea/pull/34139/files#r2040159364

[wxiaoguang]

Found various legacy (related) problems, need to fix together. If I open a separate refactoring PR, the PRs would conflict largely. So I made the changes in this PR directly. 🙏

[denyskon]

@wxiaoguang Looks good, I already noticed that the mechanism for getting the ref was flawed. I also missed that base64 encoding increases the size - I adapted the test to it. Could you fix the lint issues in your bugfix?

[wxiaoguang]

One more thing, it's impossible to make "POST" work with "read" permission at the moment, so I also added the "GET" method support. It is somewhat hacky but I think it is the only thing we can do at the moment.