[feature] For video attachments, store + return fps, bitrate, duration

[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 PR reworks some of our mp4 parsing logic, to also derive video bit rate, duration, and framerate information. This is stored in the database and then serialized to clients via the API.

This brings GtS closer in line with the Mastodon API, which also serves these fields via the client API. This should hopefully resolve some issues where videos weren't playing in some clients.

Also adds tests for the above :)

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.

[NyaaaWhatsUpDoc]

@tsmethurst have left some comments but after those are fixed up it looks good to me :)