Allow specifying key length in PKI plugin role

[bengesoff]

There is some role config allowed in the optional_config block but it doesn't cover all of the options permitted on a role. The Vault API docs have a more complete list of the available options.

The private key length and type would be a particularly pertinent option to add here given that the Venafi policy can explicitly define a rule that the Vault PKI default (RSA-2048bit) doesn't satisfy.