Handle a DCR (/register) request: persist the exact client
redirect_uris into the registry (so a later /authorize can be checked
against them) and return the registration response. The fixed upstream
client_id is shared across clients, so the registry is keyed by a
server-issued registration handle rather than that shared id.
Handle a DCR (/register) request: persist the exact client redirect_uris into the registry (so a later /authorize can be checked against them) and return the registration response. The fixed upstream client_id is shared across clients, so the registry is keyed by a server-issued registration handle rather than that shared id.