Propagate "create if missing"-flag outside binary Update payload in protocols

[vekterli]

@geirst please review. Most added lines are tests. @baldersheim FYI

Avoids potentially having to deserialize the entire update just to get to a single bit of information that is technically metadata existing orthogonally to the document update itself.

To ensure backwards/forwards compatibility, the flag is propagated as a Protobuf enum where the default value is a special "unspecified" sentinel, implying an old sender.

Since the Java protocol implementation always eagerly deserializes messages, it unconditionally assigns the create_if_missing field when sending and completely ignores it when receiving.

The C++ protocol implementation observes and propagates the field iff set. Otherwise the flag is deferred to the update object as before. This applies to both the DocumentAPI and StorageAPI protocols.