For now the config structs and validation are left as-is and
the LoginByToken method is used if the username value is PEM encoded.
In this case of username field configured with the public key, the password
field is expected to be configured with the private key.
In a follow-up PR we can look at collapsing the auth related fields into
a common struct to avoid duplication of field merging and validation.
And then add separate fields for the public and private keys.
Fixes#63209
This refactor is in support of SAML token authentication: #63209
Avoid use of govmomi.Client as it only supports username+password authentication via SessionManager.Login().
Using vim25.Client directly will allow VCP to add other authentication methods,
such as SessionManager.LoginByToken().
