feat: register flow node

[fengjiachun]

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Refer to a related PR or issue link (optional)

What's changed and what's your intention?

This PR:

1. Refactor datanode keys
2. Keep flownode's lease info in metasrv leader
3. Flownode selector

Checklist

• [ ] I have written the necessary rustdoc comments.
• [ ] I have added the necessary unit tests and integration tests.
• [ ] This PR requires documentation updates.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 74.35897% with 70 lines in your changes missing coverage. Please review.

Project coverage is 84.84%. Comparing base (ea2d067) to head (42cb006). Report is 3 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #4166 +/- ## ========================================== - Coverage 85.16% 84.84% -0.33% ========================================== Files 1021 1022 +1 Lines 179241 179735 +494 ========================================== - Hits 152650 152489 -161 - Misses 26591 27246 +655 ```