Support tidb-operator and dev tier

[xhebox]

• docker: add docker support #70
• *: adapt needs for operator support #76
• *: new tls configuration for operator #80
• *: add metrics to prometheus #79 metrics are wrong! This metrics is github.com/pingcap/tidb/metrics.
  https://github.com/pingcap/TiProxy/blob/c4d3fa9003ef95444d6a7843fed19a8c50cf27c5/pkg/proxy/proxy.go#L84
• *: ctl TLS refine #98
• initial support for tiproxy [1/2] tidb-operator#4711
• initial support for tiproxy [2/2] tidb-operator#4693
• *: support clustering #112
• proxy: enable non-TLS connection to TiDB #117

[xhebox]

Close we have full support for operator and dev-tier(not merged though).