feat: Basic Definitions for Expression&Functions for Dataflow

[discord9]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

Added following types that are worth noting:

• ScalarExpr: expressions in dataflow framework

• UnaryFunc, BinaryFunc, VariadicFunc & UnmaterializableFunc: function used in the ScalarExpr

• AggregateExpr&AggregateFunc: describing aggregate function and expressions, including basic function for accumlatoring aggregate results

• MapFilterProject: map, filter and project, including temporal filter that compare with now() system time.

• Summarize your change
  Basically a lot of utility function&types to support dataflow computation

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

Attention: 13 lines in your changes are missing coverage. Please review.

Comparison is base (0a9361a) 85.63% compared to head (487bc38) 85.65%.
Report is 9 commits behind head on main.

Additional details and impacted files

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##             main    #3267      +/-   ##
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+ Coverage   85.63%   85.65%   +0.01%     
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  Files         859      865       +6     
  Lines      140515   140798     +283     
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+ Hits       120333   120598     +265     
- Misses      20182    20200      +18     

[killme2008]

Please associate this PR with the tracking issue task.

[fengjiachun]

LGTM

[fengjiachun]

CI is failed @discord9