Support default precision and scale toCAST <EXPR> AS DECIMAL

[gandronchik]

Hello! I am an employee of Cube.js. We use cast to0 decimal with default params. Thank you!

Which issue does this PR close?

Not exists

Rationale for this change

Postgres allow to cast to decimal in the following ways:

• CAST(10 as Decimal(38, 10))
• CAST(10 as Decimal(38))
• CAST(10 as Decimal)

What changes are included in this PR?

• Cast to decimal with default params

Are there any user-facing changes?

Ability to cast to decimal with default params

[codecov-commenter]

Codecov Report

Merging #2680 (12b3faf) into master (49e072a) will increase coverage by 0.03%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #2680      +/-   ##
==========================================
+ Coverage   84.63%   84.66%   +0.03%     
==========================================
  Files         267      270       +3     
  Lines       46867    46921      +54     
==========================================
+ Hits        39665    39726      +61     
+ Misses       7202     7195       -7     

Impacted Files	Coverage Δ
datafusion/sql/src/utils.rs	51.17% <71.42%> (+0.19%)	⬆️
datafusion/sql/src/planner.rs	81.53% <100.00%> (+0.05%)	⬆️
datafusion/core/src/logical_plan/plan.rs	84.61% <0.00%> (-9.19%)	⬇️
datafusion/core/src/test/mod.rs	83.48% <0.00%> (-4.55%)	⬇️
datafusion/core/src/physical_plan/planner.rs	77.51% <0.00%> (-1.82%)	⬇️
...afusion/core/src/datasource/file_format/parquet.rs	87.32% <0.00%> (-1.24%)	⬇️
...physical-expr/src/expressions/get_indexed_field.rs	93.95% <0.00%> (-0.94%)	⬇️
datafusion/core/src/execution/memory_manager.rs	70.25% <0.00%> (-0.63%)	⬇️
datafusion/core/src/catalog/schema.rs	85.00% <0.00%> (-0.49%)	⬇️
datafusion/common/src/scalar.rs	74.82% <0.00%> (-0.36%)	⬇️
... and 76 more

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[alamb]

@gandronchik can you please provide a description of this PR (namely what problem it is solving)? It is likely obvious to you but it isn't immediately obvious to me and it is much faster to review a PR (and get it merged) if we don't have to spend time researching the "why this change" and instead can focus on "is the change in behavior desired" and "does the code implemented the desired change in behavior"

🙏 thank you

[gandronchik]

@gandronchik can you please provide a description of this PR (namely what problem it is solving)? It is likely obvious to you but it isn't immediately obvious to me and it is much faster to review a PR (and get it merged) if we don't have to spend time researching the "why this change" and instead can focus on "is the change in behavior desired" and "does the code implemented the desired change in behavior"

🙏 thank you

Sure, I will do it next time. I'm sorry for the inconvenience 🙏

[alamb]

Thanks @gandronchik -- the description makes it much easier to understand what this PR was attempting to do.

[alamb]

Looks great -- thanks @gandronchik