Add #[swift_bridge(Sendable)] attribute

[chinedufn]

This commit introduces a new #[swift_bridge(Sendable)]
attribute that emits Swift Sendable implementations for opaque
Rust types, and Rust Send + Sync implementations for opaque
Swift types.

These implementations are checked at compile time.

Example usage:

mod ffi {
    extern "Rust" {
        #[swift_bridge(Sendable)]
        type MyRustType;
    }

    extern "Swift" {
        #[swift_bridge(Sendable)]
        type MySwiftType;
    }
}