Add support for a KV V2 Secret Metadata resource

[vinay-gopalan]

Adds support for adding custom metadata to a KV V2 secret resource.

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccKVSecretV2'
=== RUN   TestAccKVSecretV2
--- PASS: TestAccKVSecretV2 (2.35s)
PASS
...

[joao-oneill-unbabel]

@vinay-gopalan Seems like this is the same as #1239

[vinay-gopalan]

@vinay-gopalan Seems like this is the same as #1239

@joao-oneill-unbabel the two are definitely similar! Though the code in this PR slightly differs from the one you mentioned, in that this resource is compatible with the latest dedicated KV V2 Secret engine support that was added to the TFVP in v3.7.0 as opposed to the legacy resource vault_generic_secret 😄

[benashz]

Looking good. Have some initial feedback for you.

[benashz]

Should this ever be a computed value? Presumably the default is empty.

[vinay-gopalan]

I wanted to make sure we are able to import the custom metadata and set it to the TF state for the KV secret, specifically in the cases that the secret metadata was created outside of TF and via Vault. That was my thought process around setting this field to also be possibly computed. Is that overkill/unnecessary?

[vinay-gopalan]

I removed the Computed from the individual nested fields, but still set Computed: true for the parent custom_metadata field

[benashz]

A few minor nits to address, then 👍

[vinay-gopalan]

1 acceptance test suite is failing due to GH request rate limiting; proceeding to merge since the work done here does not affect the failing tests