[VAULT-3157] Move mergeStates utils from Agent to api module

[vinay-gopalan]

This PR moves the utils mergeStates, compareStates from api_proxy.go in Agent to client.go in the API module. In addition, ParseRequiredStates from the hashicorp/vault/vault package is also moved to client.go. The ParseRequiredStates and mergeStates methods are also made public.

These methods need to be accessed by the Terraform Vault Provider in order to provide consistent X-Vault-Index header support.

[jasonodonnell]

LGTM!

[calvn]

Few minor nits, but otherwise looks good!

[ncabatoff]

Better make sure this works on Enterprise too. ParseRequiredState is used in some ent-specific files.

[vinay-gopalan]

Better make sure this works on Enterprise too. ParseRequiredState is used in some ent-specific files.

@ncabatoff the last spot that needs to be updated in Vault Enterprise is here in core_util_ent.go: https://github.com/hashicorp/vault-enterprise/blob/main/vault/core_util_ent.go#L1271

I'll open a PR on the vault-enterprise side by importing the api package in that file and updating that line once this PR is merged in and the changes available in the enterprise repo. Does that sound reasonable?

[ncabatoff]

Better make sure this works on Enterprise too. ParseRequiredState is used in some ent-specific files.

@ncabatoff the last spot that needs to be updated in Vault Enterprise is here in core_util_ent.go: https://github.com/hashicorp/vault-enterprise/blob/main/vault/core_util_ent.go#L1271

I'll open a PR on the vault-enterprise side by importing the api package in that file and updating that line once this PR is merged in and the changes available in the enterprise repo. Does that sound reasonable?

Yup, that's the usual practice. Just don't want you to be caught unawares and scrambling to fix the build.