swift: Initial audit for Sendable

- All public Swift-defined structs except CdsiLookupResponse,
  which wraps LookupResponseEntryList, which can *probably* be made
  Sendable but I didn't spend time on it.

- All public Swift-defined enums except those being used purely as
  namespaces.

- All zkgroup types, since they are immutable and can be serialized
  and deserialized to send them anyway

- ServiceId and its subclasses, an immutable class hierarchy

- ProtocolAddress, PinHash, SenderCertificate and ServerCertificate,
  and all public and private Key types, immutable wrappers around
  immutable Rust objects

More of our wrapper types could be made Sendable as well if there's a
need to. See CODING_GUIDELINES for more info.

swift/Sources/LibSignalClient/IncrementalMac.swift

@@ -6,7 +6,7 @@
 import Foundation
 import SignalFfi
 
-public enum SizeChoice {
+public enum SizeChoice: Sendable {
     case bytes(UInt32)
     case chunkOf(UInt32)