Add attribute for Identifiable protocol

[chinedufn]

This commit introduces the #[swift_bridge(Identifiable)] attribute
which is used to generate an Identifiable protocol implementation
for a type.

For example:

// Rust

#[swift_bridge::bridge]
mod ffi {
    extern "Rust" {
        type SomeType;

        #[swift_bridge(Identifiable, swift_name = "someFunction")]
        fn some_function(&self) -> i16;
    }
}

// Generated Swift
// (rough example, the real generated code looks a little different)

class SomeType {
    // ...
}
extension SomeType: Identifiable {
    var id: UInt16 {
        return self.someFunction()
    }
}