infosync, router: register TiProxy to ETCD

[djshow832]

What problem does this PR solve?

Issue Number: close #307

Problem Summary:
If we support self-hosted deployment, we need to support TiDB Dashboard and information_schema.cluster_xxx tables. These features require fetching TiProxy addresses. The most straightforward way is to register TiProxy in the ectd of PD, just like what TiDB does.

What is changed and how it works:

• Register info and ttl into PD by InfoSyncer
• Move InitEtcdClient from router to infosync

TODO:

• Move more etcd code from router to infosync. PDFetcher should call infosync to get TiDB topology.

Check List

Tests

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

Start TiProxy and check etcd entry from PD:

etcdctl get --prefix /topology/tiproxy
/topology/tiproxy/192.168.0.101:6000/info
{"version":"topo","git_hash":"66d42d553315fd26a3bbd677036ec211e2b49d86-dirty","ip":"192.168.0.101","port":"6000","status_port":"3080","deploy_path":"/Users/zhangming/gopath/src/github.com/pingcap/TiProxy/bin","start_timestamp":1686390578}
/topology/tiproxy/192.168.0.101:6000/ttl
1686390578299853000

And then shutdown TiProxy and check it again after 1 min:

/topology/tiproxy/192.168.0.101:6000/info
{"version":"topo","git_hash":"66d42d553315fd26a3bbd677036ec211e2b49d86-dirty","ip":"192.168.0.101","port":"6000","status_port":"3080","deploy_path":"/Users/zhangming/gopath/src/github.com/pingcap/TiProxy/bin","start_timestamp":1686390578}

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

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

None

[xhebox]

Rest LGTM