[ENG-6767 : Introduce IdentityServiceClientWrapper proxy](https://linear.app/comm/issue/ENG-6767/introduce-identityserviceclientwrapper-proxy)
Adds IdentityServiceClientSharedProxy which proxies the identity service request to the shared worker. Each method gets a new worker request message and if it returns a value also a response message. IdentityServiceContextProvider can now optionally (behind a flag) return the proxy instead of the "real" identity client.
Depends on D11274