fix: ease the restriction of the original "SelectExpr"

[MichaelScofield]

fix: ease the restriction of the original "SelectExpr" since we used to pass SQLs other than selection in the related GRPC interface

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

fix #770 (comment)

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 #794 (f4b89d8) into develop (3978931) will decrease coverage by 0.10%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #794      +/-   ##
===========================================
- Coverage    85.47%   85.37%   -0.11%     
===========================================
  Files          417      417              
  Lines        54302    54297       -5     
===========================================
- Hits         46414    46355      -59     
- Misses        7888     7942      +54     

Flag	Coverage Δ
rust	85.37% <100.00%> (-0.11%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/client/src/database.rs	70.22% <100.00%> (ø)
src/datanode/src/instance/flight.rs	33.76% <100.00%> (+0.83%)	⬆️
src/datanode/src/instance/grpc.rs	83.94% <100.00%> (ø)
src/frontend/src/instance.rs	77.79% <100.00%> (ø)
src/object-store/src/test_util.rs	0.00% <0.00%> (-100.00%)	⬇️
tests-integration/src/test_util.rs	87.98% <0.00%> (-11.54%)	⬇️
src/datanode/src/instance.rs	32.40% <0.00%> (-11.18%)	⬇️
src/servers/src/error.rs	45.23% <0.00%> (-7.15%)	⬇️
src/servers/src/grpc.rs	91.17% <0.00%> (-2.95%)	⬇️
src/servers/src/opentsdb.rs	96.82% <0.00%> (-1.59%)	⬇️
... and 6 more

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[waynexia]

Looks good to me

[killme2008]

LGTM