[WIP] Implement secret manager datasource

[ichbinfrog]

Implements the secret manager version datasource:

data "google-compute-secret-manager" "test" {
  project = "my-secret"
  name = "my-project"
  version = "1"
}

This implementation is similar to the suggested reference, however stripped down to the bare minimum (no json key decoding).

This is my first PR to this repo, so I have a few open questions:

• Since I don't see many acceptance tests in the repo, I've only implemented unit tests. I'm open to implementing it if anyone can give me an example / model to follow.

• Is there a way to "ignore" certain field in the doc generation? The mock field is not particularly useful for the enduser but mandatory (to my knowledge) to pass extra client options for unit tests.

Closes #29