Parameter to override a field type on a per-language basis

[snowsignal]

Resolves #22.

There is now a field decorator that can override the type of the field in the generated language(s) of your choice. This override takes precedent over everything else, including type mapping.

[snowsignal]

This PR depends on #118.

[Lucretiel]

General questions:

• Is this the first typeshare attribute (aside from the top-level one)?
• Is there an assumption here that the types should be compatible without any additional (de)serialize logic, in either direction, in either rust or the target language?

[snowsignal]

General questions:

* Is this the first `typeshare` attribute (aside from the top-level one)?

* Is there an assumption here that the types should be compatible without any additional (de)serialize logic, in either direction, in either rust or the target language?

• No, we already have field decorators like #[typeshare(typescript(readonly))].
• Correct, we don't check for compatibility. That's the responsibility of the end-user (given that this is usually a last-resort option)

[Lucretiel]

Slightly screwed up merging #118 so I force-rebased this branch. Merge should be fine.