[Identity] Rename IdentityService service to IdentityKeyserverService

Summary:
We are onboarding a client <-> identity gRPC service, and in order
to distinguish between the existing keyserver <-> identity gRPC service it
would be great for the identity service reflect the scenario
it's being used in.

Part of https://linear.app/comm/issue/ENG-3264

Test Plan:

nix develop

comm-dev services start # start localstack

# Configure "aws" resources on localstack
(cd services/terraform && ./run.sh)

# start identity service and run mobile workflow
(cd services/identity && cargo run -- server &)
(cd keyserver && yarn dev &)
(cd native && yarn dev &)

Then you should be able to create a new user, update the password,
and delete a user with the actions being reflected in dynamodb.
Output of the actions should also be appearing in identity logs.

Reviewers: varun, bartek, ashoat

Reviewed By: varun, ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7001