proxy: enable non-TLS connection to TiDB

[xhebox]

Signed-off-by: xhe xw897002528@gmail.com

What problem does this PR solve?

Issue Number: ref #69

Problem Summary: Dev tier has a safe private network, where TLS is unnecessary. Add requireBackendTLS to skip TLS if necessary.

What is changed and how it works:

1. proxy.require-backend-tls = true by default
2. add /pkg/sctx, which removed internal cfg.Cluster field from /lib/config/proxy.go
3. Fix wrapping errors, where Is(Wrap(e1, e2), e2) == false
4. Fix client EOF error supression
5. Fix docker build

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  set proxy.require-backend-tls = false, start tiproxy and you should be able to connect to tiup playground without setting auto-tls = true
• No code

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

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None