[Firestore] Field Path escaping in python doesn't match other languages

[crwilcox]

There are no FieldPath overloads in the SDK. Instead, there is a field_path() method that returns an encoded string. While the Python SDK expects fully escaped field paths,
other SDKS escape user-provided field paths by default and split on dots for this escaping.

[tseaver]

There is a bunch of work landed around field paths in #6291. This issue need more clarity on the delta between that work and the desired outcome.

[tseaver]

#6770 landed a new split_field_path helper, which tokenizes / splits a string path "correctly" (as far as the conformance tests go, anyway).

I plan to split the FieldPath class and related helpers out into their own module, and try to normalize how path strings are handled everywhere.