Add credential library resource for Vault

[malnick]

Adds the credential library resource for Vault:

resource "boundary_credential_store_vault" "example" {
    name  =  "foo"
    description = "foo bar"
    address = 127.0.0.1"
    namespace = "default"
 }

resource "boundary_credential_library_vault" "example" {
  name  = "example"
  description = "example resource"
  credential_store_id = boundary_credential_store.example.id

  vault_path = "/my/secrets"
  vault_http_method = "POST"
  vault_request_body = <<EOF
{
  "key": "Value",
}
EOF
}

Built on https://github.com/hashicorp/boundary/tree/mgaffney-vault

[talanknight]

Looks like it is going in the right direction.

[louisruch]

@malnick PTAL I exposed NewVaultTestServer in this PR hashicorp/boundary#1321
And then made these updates to get all tests to pass
@talanknight PTAL

[louisruch]

Just FYI I have now added the target cred lib Set functionality and associated tests...tests are all going green now, would be good to get a review so we can have this PR ready to merge once vault work merges to boundary@main

[talanknight]

@louisruch

[talanknight]

If aur has a different value for credentialStoreVaultTokenHmacKey than we have stored we should issue another update just on the token or maybe we should issue a read before the update. Same thing for the clientCertificate Key. Please also add a test for this.

[malnick]

Just a note that this PR is ready to go but we're waiting to rebase the go.mod on boundary:main when that gets merged next week to ensure our provider main branch isn't pointed at a Boundary feature branch.

Relies on hashicorp/boundary#1308 being merged to main.