[Keyserver/rust] Initialize gRPC with host's TLS certs

Summary:
Ensure that connections using https have the host's
CA certificates loaded prior to connecting to identity service.

https://linear.app/comm/issue/ENG-4410

Test Plan:

• Dev Workflow:
  • Start localstack and identity service
  • cd keyserver && yarn dev

• Prod Workflow:
  • export COMM_JSONCONFIG_secrets_identity_service_config="{\"identitySocketAddr\":\"https://identity.commtechnologies.org:50054\"}"
  • cd keyserver && yarn dev

In both workflows, the metadata table should be populated
with user_id and access_token

Reviewers: varun, bartek, ashoat

Reviewed By: varun, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8538