feat: implement drop flow parser

[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?

Implement Drop Flow statement parser

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 89.60000% with 13 lines in your changes are missing coverage. Please review.

Project coverage is 85.31%. Comparing base (a6a702d) to head (97500e4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3888      +/-   ##
==========================================
- Coverage   85.63%   85.31%   -0.33%     
==========================================
  Files         952      952              
  Lines      163496   163730     +234     
==========================================
- Hits       140017   139690     -327     
- Misses      23479    24040     +561     

[fengjiachun]

LGTM