backend: add onTraffic callback for serverless

[xhebox]

What problem does this PR solve?

Issue Number: close #275

Problem Summary: As title.

What is changed and how it works: It is called on every packet. So this callback must be small&fast.

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