This is the final step for supporting multiple OIDC providers concurrently.
First of all, we reorganise the config so that you can specify a list of OIDC providers, instead of a single one. Before:
oidc_config:
enabled: true
issuer: "https://oidc_provider"
# etc
After:
oidc_providers:
- idp_id: prov1
issuer: "https://oidc_provider"
- idp_id: prov2
issuer: "https://another_oidc_provider"
The old format is still grandfathered in.
With that done, it's then simply a matter of having OidcHandler instantiate a new OidcProvider for each configured provider.
