refactor: unify the execution of show stmt

[MichaelScofield]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

this PR unify the execution of show stmt of standalone and cluster to one place.

also write a document about how to implement statement in GreptimeDB. I think it helps understanding the codes.

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)

[codecov]

Codecov Report

Merging #1340 (1f2eebf) into develop (f8b6a6b) will decrease coverage by 0.54%.
The diff coverage is 69.41%.

❗ Current head 1f2eebf differs from pull request most recent head 82775f9. Consider uploading reports for the commit 82775f9 to get more accurate results

@@             Coverage Diff             @@
##           develop    #1340      +/-   ##
===========================================
- Coverage    85.96%   85.43%   -0.54%     
===========================================
  Files          506      510       +4     
  Lines        76892    76836      -56     
===========================================
- Hits         66103    65647     -456     
- Misses       10789    11189     +400     

[sunng87]

LGTM

[waynexia]

How about unifying Statement from SQL, EvalStmt from PromQL, and LogicalPlan together in QueryStatement? Maybe TQL can also be part of it.

[fengjiachun]

LGTM