Add tool for analyzing Vault API coverage

[tyrannosaurus-becks]

This PR adds a tool for analyzing the level of Vault API coverage we have, as well as what is and isn't supported.

The report it produces is like this:

/auth/kubernetes/config is in the Terraform Vault Provider multiple times
/sys/policy/{name} is in the Terraform Vault Provider multiple times
/auth/kubernetes/role/{name} is in the Terraform Vault Provider multiple times
 
16 percent coverage
70 of 429 vault paths are supported
359 of 429 vault paths are unsupported
 
SUPPORTED
    /auth/approle/login
    /auth/approle/role/{role_name}
    /auth/approle/role/{role_name}/custom-secret-id
    ...more...
 
UNSUPPORTED
    /ad/config
    /ad/creds/{name}
    /ad/roles
    ...more...

It will also flag resources or datasources that haven't gotten inventoried by causing tests to fail with useful output.

This report will now be run as part of the CI tests, so it'll be possible to view it by simply viewing the CI tests run with the latest commit. For an example, see the "Details" of the last successful build of this PR.

[malnick]

I love this new tool! Something like this would have been incredibly useful for other providers I wrote in previous roles. I left a couple of comments mostly geared towards linting and tests.

[tyrannosaurus-becks]

All feedback is addressed. Merging so other contributors can work off this code.