Offer Bigtable resource classes for the Data API.

[dbolduc]

As suggested by @coryan, we should add classes bigtable::TableResource and bigtable::Instance to represent the resource names used by the Data API. This is in a similar vein to google::cloud::Project or google::cloud::spanner::Database.

We have: https://github.com/googleapis/google-cloud-cpp/blob/ee484a2ef86d3e851602430ff72c29899b61141e/google/cloud/bigtable/resource_names.h but we can do better.

At least for the Data API, we want a Table constructor like:

Table(TableResource resource, Options options = {});

Having just a string is error prone. We cannot guarantee that the user made their string with TableName(...). Our API should make it hard/impossible for the user to give us a bad resource name:

// bad, because we cannot control how the user constructed the string.
Table(std::string table_name, Options options = {});

The same sorts of classes could be added for Cluster or Backup or AppProfile. If we want to do that, we can open another issue. (I think we don't want to do it. The Admin API already accepts strings, and it is final.)

[dbolduc]

blast. InstanceResource it is.

google-cloud-cpp/google/cloud/bigtable/instance_update_config.h

Lines 26 to 30 in 2d9a6ac

	namespace google {
	namespace cloud {
	namespace bigtable {
	GOOGLE_CLOUD_CPP_INLINE_NAMESPACE_BEGIN
	using Instance = ::google::bigtable::admin::v2::Instance;