[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

[hashicorp-cla]

All committers have signed the CLA.

[nywilken]

Hi @ichbinfrog apologies for the longer then normal review process. We haven't had the chance to prioritize this review. We have however selected the PR and will provide feedback if you are still interested in working on the change. Thanks!