Is your feature request related to a problem? Please describe.
When using the Vault library (https://pkg.go.dev/github.com/hashicorp/vault/api) as a Vault client, there is no auth method to log in to the Vault server using cert auth.

Describe the solution you'd like

Add cert auth in vault/api/auth so that applications using this library as a Vault client can log in to the Vault server using cert auth.

Describe alternatives you've considered

Currently, to make cert auth work, we implemented our own custom cert auth method conforming to the AuthMethod interface. https://github.com/hashicorp/vault/blob/main/api/auth.go#L16

Additional context

It would be great if we had built-in cert auth support in vault/api/auth so that people using this library do not need to create their own cert auth method.

old issue:
#29324