Allow custom names for indices and fields

[maroux]

• This allows one to name an index the same as a field like so:

class FieldCustomNameTestModel(model.Model):
  __table__ = 'FieldCustomNameTestModel'

  key = field.Field(field.String, primary_key=True)
  value = field.Field(field.String, nullable=False)

  value_idx = index.Index(['value'], name='value')

[dgorelik]

Hi, sorry for a delayed review. We are now trying to make use of the Cloud Spanner Emulator for testing. Can you add some test coverage which uses the Cloud Spanner Emulator? See this example: https://github.com/google/python-spanner-orm/blob/a95d8dc9c779f0aeba6158e1f799582f1924e426/spanner_orm/tests/migrations_emulator_test.py