Add CI for mobile platforms (iOS & Android)

[thomaseizinger]

Currently, quinn (and quinn-udp) is only tested on Linux, Windows and MacOS: https://github.com/quinn-rs/quinn/blob/main/.github/workflows/rust.yml#L36

There have been a number of PRs that fix things on other platforms:

• https://github.com/quinn-rs/quinn/pull/1516
• https://github.com/quinn-rs/quinn/pull/1504
• https://github.com/quinn-rs/quinn/pull/849
• https://github.com/quinn-rs/quinn/pull/635
• https://github.com/quinn-rs/quinn/pull/1758
• https://github.com/quinn-rs/quinn/pull/1788

All of those have only been validated with manual testing which isn't great for maintenance. Simple refactorings could easily break those features again.

It would be great to test the features of GRO & GSO as well as retrieval of the additional packet meta data on Android as well as iOS.