feat: Lazy load Endpoint class

[vinnysenthil]

Fixes b/194435202 🦕

[sasha-gitg]

Couple of thoughts:

1. It seems like we don't get the benefit of lazy loading because at predict time we're making the get call through predict -> self.resource_name -> self._sync_gca_resource -> self._sync_gca_resource_if_skipped

2. Perhaps a simpler implementation is to populate _gca_resource in the constructor like so:

endpoint_name = utils.full_resource_name(...)

self._gca_resource = gca_endpoint.Endpoint(name=endpoint_name)

3. Please extend the integration test to retrieve one of the created models, predict, and assert we didn't get the resource.

[sasha-gitg]

This is creating a new instance attribute but _skipped_getter_call is defined as a class attribute above. Perhaps, _skipped_getter_call should be an instance attribute?

[sasha-gitg]

Thanks Vinny! I added one comment that may simplify this change.