Make @DocumentID property wrapper conform to the Sendable Protocol

The @DocumentID property wrapper for Firestore does not conform to the concurrency protocol Sendable in the upcoming Swift 5.5. Conformance to Sendable will be very important for types that need to be used with Actors. This document provides more details: https://github.com/apple/swift-evolution/blob/main/proposals/0302-concurrent-value-and-concurrent-closures.md

We can override the compiler with @unchecked annotation on the type but I'm hesitant to do that without a confirmation that @DocumentID can in fact be safely Sendable