Started refactor for merge of OIDC

- Made oidc config more generic to not be overly reliant on the library
  based upon learnings from saml2 auth.
- Removed any settings that are redundant or not deemed required for
  initial implementation.
- Reduced some methods down where not needed.
- Renamed OpenID to OIDC
- Updated .env.example.complete to align with all options and their
  defaults

Related to #2169

.env.example.complete

@@ -240,12 +240,15 @@ SAML2_GROUP_ATTRIBUTE=group
 SAML2_REMOVE_FROM_GROUPS=false
 
 # OpenID Connect authentication configuration
-OPENID_CLIENT_ID=null
-OPENID_CLIENT_SECRET=null
-OPENID_ISSUER=https://example.com
-OPENID_PUBLIC_KEY=file:///my/public.key
-OPENID_URL_AUTHORIZE=https://example.com/authorize
-OPENID_URL_TOKEN=https://example.com/token
+OIDC_NAME=SSO
+OIDC_DISPLAY_NAME_CLAIMS=name
+OIDC_CLIENT_ID=null
+OIDC_CLIENT_SECRET=null
+OIDC_ISSUER=null
+OIDC_PUBLIC_KEY=null
+OIDC_AUTH_ENDPOINT=null
+OIDC_TOKEN_ENDPOINT=null
+OIDC_DUMP_USER_DETAILS=false
 
 # Disable default third-party services such as Gravatar and Draw.IO
 # Service-specific options will override this option