Add support for gRPC interceptors

[parthea]

interceptors are currently supported for REST transport. We should also add support for interceptors in gRPC to have feature parity between both transports.

gapic-generator-python/gapic/templates/%namespace/%name_%version/%sub/services/%service/transports/rest.py.j2

Lines 74 to 77 in 24a23a1

	class {{ service.name }}RestInterceptor:
	"""Interceptor for {{ service.name }}.
	Interceptors are used to manipulate requests, request metadata, and responses

Also see https://github.com/googleapis/python-spanner/blob/4da06a6a883c0513d7c80f9462c871b0a00a8f2b/google/cloud/spanner_v1/testing/database_test.py#L31-L35