feat(firestore): support "!=" and "not-in" query operators

Firestore Version 7.21.0 - September 17, 2020 - introduced support for
the "!=" (NOT_EQUAL) and "not-in" (NOT_IN) query operators:
https://firebase.google.com/support/release-notes/js#version_7210_-_september_17_2020

Query operator documentation:
https://firebase.google.com/docs/firestore/query-data/queries

This patch adds support for these two new operators and adds test
cases for them.

firestore/integration_test.go

@@ -651,6 +651,11 @@ func TestIntegration_QueryDocuments(t *testing.T) {
 	}{
 		{q, wants},
 		{q.Where("q", ">", 1), wants[2:]},
+		{q.Where("q", "<", 1), wants[:1]},
+		{q.Where("q", "==", 1), wants[1:2]},
+		{q.Where("q", "!=", 0), wants[1:]},
+		{q.Where("q", ">=", 1), wants[1:]},
+		{q.Where("q", "<=", 1), wants[:2]},
 		{q.WherePath([]string{"q"}, ">", 1), wants[2:]},
 		{q.Offset(1).Limit(1), wants[1:2]},
 		{q.StartAt(1), wants[1:]},

firestore/query.go

@@ -500,8 +500,12 @@ func (f filter) toProto() (*pb.StructuredQuery_Filter, error) {
 		op = pb.StructuredQuery_FieldFilter_GREATER_THAN_OR_EQUAL
 	case "==":
 		op = pb.StructuredQuery_FieldFilter_EQUAL
+	case "!=":
+		op = pb.StructuredQuery_Filter_NOT_EQUAL
 	case "in":
 		op = pb.StructuredQuery_FieldFilter_IN
+	case "not-in":
+		op = pb.StructuredQuery_Filter_NOT_IN
 	case "array-contains":
 		op = pb.StructuredQuery_FieldFilter_ARRAY_CONTAINS
 	case "array-contains-any":

firestore/query_test.go

@@ -143,11 +143,21 @@ func TestQueryToProto(t *testing.T) {
 			in:   q.Where("a", "==", float32(math.NaN())),
 			want: &pb.StructuredQuery{Where: filtr([]string{"a"}, "==", math.NaN())},
 		},
+		{
+			desc: `q.Where("a", "!=", NaN)`,
+			in:   q.Where("a", "!=", float32(math.NaN())),
+			want: &pb.StructuredQuery{Where: filtr([]string{"a"}, "!=", math.NaN())},
+		},
 		{
 			desc: `q.Where("a", "in", []int{7, 8})`,
 			in:   q.Where("a", "in", []int{7, 8}),
 			want: &pb.StructuredQuery{Where: filtr([]string{"a"}, "in", []int{7, 8})},
 		},
+		{
+			desc: `q.Where("a", "not-in", []int{9})`,
+			in:   q.Where("a", "not-in", []int{9}),
+			want: &pb.StructuredQuery{Where: filtr([]string{"a"}, "not-in", []int{9})},
+		},
 		{
 			desc: `q.Where("c", "array-contains", 1)`,
 			in:   q.Where("c", "array-contains", 1),