refactor: changes CreateTableRequest::schema to RawSchema

[evenyag]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

This PR use RawSchema as the type of CreateTableRequest::schema to make it easier to serialize and deserialize contents of CreateTableRequest.

This is useful for implementing a procedure that creates a table because it needs to serialize and deserialize the request and pass the request to the table engine. The subprocedure created by the table engine may again need to serialize it.

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)

• Tracking issue for the procedure framework #286

[codecov]

Codecov Report

Merging #1018 (8d1e3b8) into develop (a8c2b35) will decrease coverage by 0.28%.
The diff coverage is 98.80%.

@@             Coverage Diff             @@
##           develop    #1018      +/-   ##
===========================================
- Coverage    86.23%   85.95%   -0.28%     
===========================================
  Files          436      436              
  Lines        63279    63269      -10     
===========================================
- Hits         54568    54386     -182     
- Misses        8711     8883     +172     

Flag	Coverage Δ
rust	85.95% <98.80%> (-0.28%)	⬇️

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

Impacted Files	Coverage Δ
src/catalog/src/remote/manager.rs	74.09% <0.00%> (+0.95%)	⬆️
src/table/src/requests.rs	66.66% <ø> (ø)
src/catalog/src/system.rs	94.76% <100.00%> (-0.02%)	⬇️
src/common/grpc-expr/src/alter.rs	94.14% <100.00%> (-0.04%)	⬇️
src/common/substrait/src/df_logical.rs	77.02% <100.00%> (-0.88%)	⬇️
src/datanode/src/server/grpc.rs	98.82% <100.00%> (-0.03%)	⬇️
src/datanode/src/sql/create.rs	90.26% <100.00%> (-0.52%)	⬇️
src/datatypes/src/schema/raw.rs	100.00% <100.00%> (ø)
src/mito/src/engine.rs	98.38% <100.00%> (-0.02%)	⬇️
src/mito/src/table/test_util.rs	100.00% <100.00%> (ø)
... and 15 more

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