backend: keep backend TLS config consistent with frontend TLS config

[djshow832]

What problem does this PR solve?

Issue Number: close #302

Problem Summary:
Currently, the TLS config between client and TiProxy is specified by the client, the TLS between TiProxy and TiDB is always disabled.

In the current implementation, the TLS config between TiProxy and TiDB is specified by TiProxy config. However, what we want is to keep consistent with client -> TiProxy. That is:

• If client->TiProxy is enabled, TiProxy -> TiDB is also enabled
• If client->TiProxy is disabled, TiProxy -> TiDB is also disabled

What is changed and how it works:
Enable TLS between TiProxy and TiDB when:

• client enables TLS
• proxy configured TLS certs.
• backend supports TLS

This is compatible with both serverless and dedicated tiers.

However, this makes RequireBackendTLS sometimes useless: it requires TLS from the backend but it doesn't need TLS.

Check List

Tests

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

Notable changes

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

Release note

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None