Add support for != and not-in queries

[HemangChothani]

Porting from Java and Node

For python auto generated code is missing here whereas it's generated for java.

python-firestore/google/cloud/firestore_v1/types/query.py

Lines 173 to 183 in 581520c

	class Operator(proto.Enum):
	r"""A field filter operator."""
	OPERATOR_UNSPECIFIED = 0
	LESS_THAN = 1
	LESS_THAN_OR_EQUAL = 2
	GREATER_THAN = 3
	GREATER_THAN_OR_EQUAL = 4
	EQUAL = 5
	ARRAY_CONTAINS = 7
	IN = 8
	ARRAY_CONTAINS_ANY = 9

@tseaver @crwilcox Should i run synth.py for that?

[HemangChothani]

Found the PR of auto generated code #171 and it's blocked for now.

[tseaver]

PR #187 replaced PR #171. It added NOT_EQUAL and NOT_IN:

        class Operator(proto.Enum):
            r"""A field filter operator."""
            OPERATOR_UNSPECIFIED = 0
            LESS_THAN = 1
            LESS_THAN_OR_EQUAL = 2
            GREATER_THAN = 3
            GREATER_THAN_OR_EQUAL = 4
            EQUAL = 5
            NOT_EQUAL = 6
            ARRAY_CONTAINS = 7
            IN = 8
            ARRAY_CONTAINS_ANY = 9
            NOT_IN = 10