[Firestore] Add conditonal Sendability conformance to ServerTimestamp (#14162)

Author: ncooke3

Firestore/CHANGELOG.md

@@ -1,3 +1,7 @@
+# Unreleased
+- [fixed] Add conditional `Sendable` conformance so `ServerTimestamp<T>` is
+  `Sendable` if `T` is `Sendable`. (#14042)
+
 # 11.4.0
 - [changed] Prepare Firestore cache to support session token.
 

Firestore/Swift/Source/Codable/ServerTimestamp.swift

@@ -105,3 +105,5 @@ public struct ServerTimestamp<Value>: Codable
 extension ServerTimestamp: Equatable where Value: Equatable {}
 
 extension ServerTimestamp: Hashable where Value: Hashable {}
+
+extension ServerTimestamp: Sendable where Value: Sendable {}