Open classes in uniffi rust bindings

[stefanceriu]

One of the biggest problems we have in testing at the moment is that we cannot mock Rust side objects, both because they're not using protocols throughout and because they're not open.

Following the great work @jmartinesp did here we would like to make something similar available to swift developers. We should then be able to unit test more complex services layer files like the RoomTimelineItemFactory and RoomSummaryProvider

[nimau]

The following PRs have been opened:

• uniffi PR to build Swift mocks using uniffi
• draft matrix-rust-sdk PR, which updates the Swift xtask to generate mocks when building the framework
• draft element-x-ios PR, which updates the BuildSDK tool and adds some unit tests for ClientProxy that demonstrate how to use the generated Swift mocks.

[ara4n]

Out of interest, has a mock-less approach been considered? cf https://accu.org/journals/overload/23/127/balaam_2108/ and https://cs-syd.eu/posts/2021-10-22-why-mocking-is-a-bad-idea and similar

[stefanceriu]

This is now done in uniffi but without automocking (which they didn't accept) mozilla/uniffi-rs#1975