Adding support for acr_values configuration of oidc

[matya]

The oidc specification details the use of ACR Values that can optionally be provided for an auth request.

• https://openid.net/specs/openid-connect-core-1_0.html#AuthRequest

This is often coupled with specifying critical requirements to the auth process, like the use of MFA.

• https://developer.okta.com/docs/guides/step-up-authentication/main/

There have been periodic questions in the community about the availability of this feature:

• https://discuss.hashicorp.com/t/acr-values-support-with-oidc-provider-configuration/11826/2
• https://discuss.hashicorp.com/t/mfa-with-oidc-auth-configuration-of-authorization-oauth2-acr-values-xxyyzz/21628

After reviewing the code and following up the code-path laid out by eliasp in the second discussion, it is not too complicated to enable the use of the already existing feature within the hashicorp/cap package, internally used by this plugin, it only requires handling the more or less trivial configuration field, the heavy lifting is already implemented.

I am opening this issue to propose a PR with the mentioned feature and coordinate any possible feedback / recommendation / modification requirements.