feat: add migration start step

[WenyXu]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

Add migration start step

• If the expected leader region has been opened on to_peer, go to the MigrationEnd state.
• If the candidate region has been opened on to_peer, go to the DowngradeLeader state.
• Else go to the OpenCandidateRegion state.

Behaviors:

Abort(non-retry):

• TableRoute is not found.
• RegionRoute is not found.
• Leader peer of RegionRoute is not found.

Retry:

• Failed to retrieve the metadata of table.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.

Refer to a related PR or issue link (optional)

#2700

[codecov]

Codecov Report

Merging #2756 (a2cba0f) into develop (9bd1013) will decrease coverage by 0.44%.
Report is 2 commits behind head on develop.
The diff coverage is 89.67%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2756      +/-   ##
===========================================
- Coverage    85.46%   85.02%   -0.44%     
===========================================
  Files          772      777       +5     
  Lines       125283   126039     +756     
===========================================
+ Hits        107069   107161      +92     
- Misses       18214    18878     +664     

[WenyXu]

@MichaelScofield @fengjiachun PTAL

[WenyXu]

@fengjiachun PTAL

[fengjiachun]

LGTM