feat(datastore): Add read time from client while running queries

[bhshkh]

Fixes: #11012

User can create a client with read time option. This option wasn't getting passed in RunQueryRequest and RunAggregationQueryRequest. This PR adds this option.

google-cloud-go/datastore/apiv1/datastorepb/datastore.pb.go

Lines 464 to 465 in 93e313d

	// The options for this query.
	ReadOptions *ReadOptions `protobuf:"bytes,1,opt,name=read_options,json=readOptions,proto3" json:"read_options,omitempty"`

google-cloud-go/datastore/apiv1/datastorepb/datastore.pb.go

Lines 701 to 702 in 93e313d

	// The options for this query.
	ReadOptions *ReadOptions `protobuf:"bytes,1,opt,name=read_options,json=readOptions,proto3" json:"read_options,omitempty"`

google-cloud-go/datastore/apiv1/datastorepb/datastore.pb.go

Lines 2184 to 2205 in 93e313d

	// The options shared by read requests.
	type ReadOptions struct {
	state protoimpl.MessageState
	sizeCache protoimpl.SizeCache
	unknownFields protoimpl.UnknownFields
	// For Cloud Firestore in Datastore mode, if you don't specify
	// read_consistency then all lookups and queries default to
	// `read_consistency`=`STRONG`. Note that, in Cloud Datastore, global queries
	// defaulted to `read_consistency`=`EVENTUAL`.
	//
	// Explicitly setting `read_consistency`=`EVENTUAL` will result in eventually
	// consistent lookups and queries.
	//
	// Types that are assignable to ConsistencyType:
	//
	// *ReadOptions_ReadConsistency_
	// *ReadOptions_Transaction
	// *ReadOptions_NewTransaction
	// *ReadOptions_ReadTime
	ConsistencyType isReadOptions_ConsistencyType `protobuf_oneof:"consistency_type"`
	}

As can be seen in above proto, the consistency type field in ReadOptions can be one of ReadOptions_ReadConsistency_, ReadOptions_Transaction, ReadOptions_NewTransaction or ReadOptions_ReadTime. Same validation has been added on client