Nextgen Proto Pythonic API: Add 'in' operator

The “in” operator will be consistent with HasField but a little different with Proto Plus.

The detail behavior of “in” operator in Nextgen for Struct (to be consist with old Struct behavior):
-Raise TypeError if not pass a string
-Check if the key is in the struct.fields

The detail behavior of “in” operator in Nextgen(for other message):
-Raise ValueError if not pass a string
-Raise ValueError if the string is not a field
-For Oneof: Check any field under the oneof is set
-For has-presence field: check if set
-For non-has-presence field (include repeated fields): raise ValueError

PiperOrigin-RevId: 621240977