refactor: remove optional from the protos

[fengjiachun]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

Remove optional from protos, because the optional relies on a higher version of proto (supported in 3.15.0), which often causes some problems for users, e.g. (#616 #737 ), and I think we can avoid using this feature.

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)

#616 #737

[codecov]

Codecov Report

Merging #756 (ff49646) into develop (28bd740) will increase coverage by 0.03%.
The diff coverage is 71.34%.

❗ Current head ff49646 differs from pull request most recent head 404ef38. Consider uploading reports for the commit 404ef38 to get more accurate results

@@             Coverage Diff             @@
##           develop     #756      +/-   ##
===========================================
+ Coverage    85.39%   85.43%   +0.03%     
===========================================
  Files          413      413              
  Lines        54142    54165      +23     
===========================================
+ Hits         46237    46278      +41     
+ Misses        7905     7887      -18     

Flag	Coverage Δ
rust	85.43% <71.34%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
benchmarks/src/bin/nyc-taxi.rs	0.29% <0.00%> (ø)
src/sql/src/statements/alter.rs	31.57% <0.00%> (ø)
src/frontend/src/expr_factory.rs	87.06% <60.00%> (-0.76%)	⬇️
src/frontend/src/instance/distributed.rs	80.82% <62.50%> (-1.33%)	⬇️
src/api/src/v1/column_def.rs	100.00% <100.00%> (ø)
src/client/src/admin.rs	62.10% <100.00%> (ø)
src/common/grpc-expr/src/alter.rs	97.84% <100.00%> (+0.18%)	⬆️
src/common/grpc-expr/src/insert.rs	82.77% <100.00%> (ø)
src/datanode/src/instance/grpc.rs	80.45% <100.00%> (+0.14%)	⬆️
src/datanode/src/server/grpc.rs	81.30% <100.00%> (+0.16%)	⬆️
... and 15 more

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[fengys1996]

LGTM