Add ability to save resources

[parafoxia]

Summary

This PR adds the ability to save attachments (and more generally, resources) to disk.

• Adds the Resource.save() method
• Adds tests for Resource.save() and Resource.read()

Checklist

• I have run nox and all the pipelines have passed.
• I have made unittests according to the code I have added/modified/deleted.

Related issues

None

[parafoxia]

Honestly I think I'm just gonna have to leave the tests up to somebody else to take care of cos I can't figure this out. I've emulated what you've done, and I'm getting all sorts of errors and have absolutely no idea what I'm doing wrong.

[davfsa]

Honestly I think I'm just gonna have to leave the tests up to somebody else to take care of cos I can't figure this out. I've emulated what you've done, and I'm getting all sorts of errors and have absolutely no idea what I'm doing wrong.

@parafoxia mind pushing what you have (can be with broken tests too, or just skip them entirely).

[parafoxia]

There's not much extra there, but that's where I got up to. The error it's giving is that the async context manager mock can't be used in async contexts.

[parafoxia]

@FasterSpeeding, just bumping this as it's ready for review.

[FasterSpeeding]

Test cases for the specialisations?

[davfsa]

Test cases for the specialisations?

Working on those atm :)

[davfsa]

@FasterSpeeding tests done. I also found some bugs in the code, so some parts are update too :)