Add check for valid PEM format / certificate

When adding a CF auth backend, Vault will verify the credentials. I've hit a scenario where an invalid CA cert was entered, but it isn't validated till an app attempts to authenticate. This resulted in the following:

https://github.com/hashicorp/vault-plugin-auth-cf/blob/335c793bf24c79b2e12d82eb84ac9ad90cae3ff3/util/certificates.go#L66

Could there be a check where when enabling the auth method backend, Vault validates the `identity_ca_certificates` values and the `cf_api_trusted_certificates` values?