Add a CI pipeline that builds with the explicit sendable setting

[czechboy0]

It's good practice to mark all public types as explicit Sendable or non-sendable, instead of leaving it implicit.

This can be enforced in CI by adding e.g.:

swiftSettings.append(.unsafeFlags(["-Xfrontend", "-require-explicit-sendable"]))

We should either add these flags to an existing pipeline and promote them to errors, or add a separate pipeline.