Classify using session prefix

For gitlab-org/gitlab#465433 (closed)

The changes in this MR supports

• introducing attribute named session_prefix in config.toml
• classification using a session's prefix.
  • A session's prefix matches a cell's session_prefix attribute specified in the configuration, so we match this value (which HTTP router would pass to TS) in the request to the session_prefix attributes in the TS's configuration to find the right cell to route the request to.

With a valid cell name

curl --cacert tmp/certs/ca-cert.pem --cert tmp/certs/client-cert.pem --key tmp/certs/client-key.pem  -X POST https://localhost:9096/v1/classify -H "Content-Type: application/json"   -d '{"type": "SessionPrefix", "value": "cell-1"}'

{"action":"Proxy","proxy":{"address":"my.cell-1.example.com"}}

With an invalid cell name

 curl --cacert tmp/certs/ca-cert.pem --cert tmp/certs/client-cert.pem --key tmp/certs/client-key.pem  -X POST https://localhost:9096/v1/classify -H "Content-Type: application/json"   -d '{"type": "SessionPrefix", "value": "junk_value"}'

{"code":5,"message":"cell not found: \"junk_value\"","details":[]}