feat: define CreateFlowTask and DropFlowTask

[WenyXu]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

What's changed and what's your intention?

Define CreateFlowTask and DropFlowTask.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

[codecov]

Codecov Report

Attention: Patch coverage is 6.00000% with 94 lines in your changes are missing coverage. Please review.

Project coverage is 85.30%. Comparing base (2d0f493) to head (cacd886).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3801      +/-   ##
==========================================
- Coverage   85.62%   85.30%   -0.32%     
==========================================
  Files         947      953       +6     
  Lines      161925   162881     +956     
==========================================
+ Hits       138647   138953     +306     
- Misses      23278    23928     +650     

[fengjiachun]

LGTM