Nextgen Proto Pythonic API: Add 'in' operator

(Second attempt. The first attempt missed ListValue)

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

The detail behavior of “in” operator in Nextgen

* For WKT Struct (to be consist with old Struct behavior):
    -Raise TypeError if not pass a string
    -Check if the key is in the struct.fields

* For WKT ListValue (to be consist with old behavior):
    -Check if the key is in the list_value.values

* For other messages:
    -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: 631143378