backend: add namespace/token to the log of BackendConnManager

[djshow832]

What problem does this PR solve?

Issue Number: close #240

Problem Summary:
Add namespace/token to the log of BackendConnManager to help debug.

What is changed and how it works:
Add UpdateLogger to ConnContext so that the HandshakeHandler can add customized fields.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

[2023/03/07 17:07:48.831 +08:00] [ERROR] [main.proxy.conn.be] [backend/backend_conn_mgr.go:258] [get backend failed] [connID=2] [remoteAddr=127.0.0.1:51124] [ns=default] [duration=15.000067206s] [last_err="no instances to route"]

Notable changes

• Has configuration change
• Has HTTP API interfaces change (Don't forget to add the declarative for API)
• Has tiproxyctl change
• Other user behavior changes

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None