This PR implements methods to retrieve the region in which the current cloud instance is running. Structurally, these methods are analogous to projectId supporting methods, found within gcp/lib/src/gcp_project.dart

Usage of these new methods follow a similar pattern:

final computedRegion = await computeRegion();
print(computedRegion);
...
// Outputs: us-central1

final environmentRegion = regionFromEnvironment();
print(environmentRegion);
...
// Outputs: us-central1

Practically, these new methods are copy-pastes, substituting projectId with region. In my original proposal within #392, I perhaps erroneously utilized location rather than region. After combing through some GCP documentation, it looks like region is perhaps the more widely utilized term?

I am looking for additional feedback on the code documentation for these methods. Most of the documentation from gcp_project.dart was brought forward, with some subjective edits to represent instance & region vs. project Ids.

Regarding the metadata server, I linked to instance metadata to provide further documentation on what these instance endpoints provide, however, I was unable to find v1/instance/region within this public documentation. Is this particular endpoint not documented there for a reason?

I tried combing through search to find various environment variables that currently exist, that could represent the instance's region. I was only successful in finding two discretely populated variables. If there are more, I would like to add them in to add some redundancy in the same way the projectId is evaluated.

Finally, in regards to unit tests, I brought forward the same pattern used to test computeProjectId. This includes the TODO comment regarding stubs for the metadata server under test.

Closes #392