[Firestore] un-constant server timestamp and delete values to support ARRAY_UNION, etc

[crwilcox]

FieldValue.serverTimestamp() and FieldValue.delete() are exposed as constants, which will become problematic as we add arrayUnion()/arrayRemove()/numericAdd().

[tseaver]

There is no FieldValue in Python. I am working now to refactor how the various transforms are handled, and do not expect to need a non-constant value for deleted fields or server timestamp-valued fields.

ArrayUnion and ArrayRemove will not be constants, but rather helper classes (see my comment in #6546).

We have no conformance tests for any numeric_add transform, and I do not see any corresponding element in the FieldTransform docs.

[crwilcox]

Closing as #6546 is the work item. This will be fixed if necessary via that issue.