Enable Spanner Request Priority argument in queries / sessions

[awertman]

The standard cloud spanner python API enables a RequestOptions parameter in requests which can be used to set request Priorities.

https://cloud.google.com/python/docs/reference/spanner/latest/google.cloud.spanner_v1.types.RequestOptions
https://cloud.google.com/python/docs/reference/spanner/latest/google.cloud.spanner_v1.types.RequestOptions.Priority

https://github.com/googleapis/python-spanner/blob/474b1b56cd11bc22d274a85664b09532a576e3d6/tests/unit/test_snapshot.py#L810

We would like to be able to set low and medium priorities on certain requests.

[IlyaFaer]

@asthamohta, this should be possible. We're already using some execution options:

python-spanner-sqlalchemy/google/cloud/sqlalchemy_spanner/sqlalchemy_spanner.py

Line 145 in 68b9bc8

class SpannerExecutionContext(DefaultExecutionContext):

Something similar can be done for priorities.