proxy/net: human readable capability flag

[xhebox]

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

What problem does this PR solve?

Issue Number: close None

Problem Summary: The motivation is that I want to print capability difference between server and proxy in a human readable format. I'll modify all capability flags in the package to this type later. And maybe cherry pick the change to tidb/parser.

What is changed and how it works: A new capability flag type with fmt.Sringer and TextMarshaler.

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 (Don't forget to add the declarative for API)
• Has weirctl change
• Other user behavior changes

Release note

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None