Migration of sub claim & introspection endpoint username claim for Application tokens

[mpmadhavig]

Is your feature request related to a problem? Please describe.
This issue will track the on-the-fly migration support given to enable the below two configs without any flow breaks.

[oauth]
use_client_id_as_sub_claim_for_app_tokens = false
remove_username_from_introspection_response_for_app_tokens = false

Describe the solution you would prefer
Implement application level configs and act according the state of those configs. After a grace period, completely move to the server level config.

Related issues:

• Behaviour Change in Access Tokens issued for authenticated applications #16060