Provide support for okta auth method

[robkinyon-roivant]

For the most part, this is identical to the ldap auth method, but the mount point defaults to okta.

Note: This probably requires #197 to be addressed first.

[dylan-azucena]

@robkinyon-roivant Would this PR be what you're looking for here? If so, I'm not sure how or who to nudge for this, but I'd like to get that branch pushed through.