feat(datastore): Add query profiling

[bhshkh]

Introducing ExplainOptions:

• When ExplainOptions is not used: Datastore plans and executes the query. Only returns the query results.
• When ExplainOptions.Analyze is false: Datastore plans but does not execute the query. Only returns planner stage information. Can be used to check that a query has the necessary indexes and understand which indexes are used.
• When ExplainOptions.Analyze is true: Datastore plans and executes the query. Returns query results, planner stage information and execution statistics.

samples at https://github.com/GoogleCloudPlatform/golang-samples/pull/3742

[bhshkh]

Samples at https://github.com/GoogleCloudPlatform/golang-samples/pull/3742

[bhshkh]

Do not merge until release freeze ends in mid April