Use a type alias to allow bindings to take advantage of custom types

[zzorba]

This PR adds a new type alias to the matrix-sdk-ffi crate called Date64. This is aliased to u64 for backwards compatibility (so no change to Swift/Kotlin as is), but as a distinct type it will allow uniffi integrations to customize the unpacking of the type automatically: https://mozilla.github.io/uniffi-rs/latest/udl/custom_types.html

Using this, it is possible to make the uniffi layer automatically convert these from u64 -> an appropriate system Date class in the generated code layer.

Today a u64 type can mean either a count or a date, so without an alias implementing a customType would not be possible. At the very least, the type helps improve the documentation, even if individual bindings choose not to adopt this in their UDL configs.

Signed-off-by: Daniel Salinas

[zzorba]

I primarily want this for the react-native-matrix-sdk (Typescript), but I believe it is possible to add custom unpackers to Kotlin/Swift as well. That would of course be a breaking API change for any users of this library, the current iteration of this should fallback to u64 automatically.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.27%. Comparing base (54bd1d7) to head (69811df).
Report is 64 commits behind head on main.

Additional details and impacted files

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- Coverage   85.27%   85.27%   -0.01%     
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  Files         282      282              
  Lines       31186    31186              
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- Hits        26594    26593       -1     
- Misses       4592     4593       +1     

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[jplatte]

Small nitpick about the PR description, it talks about a marker trait but there is no new trait involved here, I think you meant "newtype"?

[Hywan]

Thanks, that's a nice improvement! I'm suggesting a renaming but everything else is nice!

[poljar]

Review comments were addressed and the PR seems fine to me as well.

Comment was addressed.