Implement linter validation result serializer

[EnTeQuAk]

In #6287 I only added a link to the linter validation result but thinking about this more I think it makes sense to actually implement a serializer that can control and adjust data as we need it.

@willdurand @bobsilverberg when touching this, does it make sense for you to inline the contents validation_url_json into the browse API directly? Otherwise, I may make this a separate endpoint to request results (instead of exposing that URL as I did) or just inline the information. On the other hand, for bigger files, and diffs, that may end up being quite a bit of content so retrieving it in a second step sounds reasonable to me.

[bobsilverberg]

As I understand it, we'll only need this information when looking at a specific file - is that correct? If so then I think it makes sense to inline it with the API response that returns the file's contents. @EnTeQuAk and I also discussed today having a separate endpoint for an individual file (which could include its contents and this validation info) apart from an endpoint that returns the file entries used to build the file tree.

[kumar303]

@EnTeQuAk we realized that we do need to get the linter result from a separate endpoint because the compare/diff view also needs to show linter messages. Thus, we can begin using the existing validation.json URL. No need to prioritize a new endpoint over that one.

[stale]

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