feat(stream): implement framework of now executor

[soundOfDestiny]

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What's changed and what's your intention?

This is framework of Now executor.
Here defines interface of Now executor and builder.
Frontend part, backend part and barrier part of now executor will be based on this.

Checklist

• I have written necessary rustdoc comments
• All checks passed in ./risedev check (or alias, ./risedev c)

Documentation

If your pull request contains user-facing changes, please specify the types of the changes, and create a release note. Otherwise, please feel free to remove this section.

Types of user-facing changes

Please keep the types that apply to your changes, and remove those that do not apply.

• Installation and deployment
• Connector (sources & sinks)
• SQL commands, functions, and operators
• RisingWave cluster configuration changes
• Other (please specify in the release note below)

Release note

Please create a release note for your changes. In the release note, focus on the impact on users, and mention the environment or conditions where the impact may occur.

Refer to a related PR or issue link (optional)

#6408

[codecov]

Codecov Report

Merging #6677 (498a719) into main (2d87383) will decrease coverage by 0.09%.
The diff coverage is 61.27%.

@@            Coverage Diff             @@
##             main    #6677      +/-   ##
==========================================
- Coverage   74.07%   73.98%   -0.10%     
==========================================
  Files         989      995       +6     
  Lines      161850   162137     +287     
==========================================
+ Hits       119888   119950      +62     
- Misses      41962    42187     +225     

Flag	Coverage Δ
rust	73.98% <61.27%> (-0.10%)	⬇️

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

Impacted Files	Coverage Δ
src/batch/src/error.rs	0.00% <ø> (ø)
src/common/src/catalog/schema.rs	78.12% <0.00%> (-6.34%)	⬇️
src/compute/src/server.rs	0.00% <0.00%> (ø)
src/expr/src/error.rs	25.00% <ø> (ø)
src/frontend/src/catalog/root_catalog.rs	72.76% <0.00%> (-2.52%)	⬇️
src/frontend/src/handler/drop_index.rs	59.70% <0.00%> (-5.88%)	⬇️
src/frontend/src/handler/drop_source.rs	0.00% <0.00%> (-85.15%)	⬇️
src/risedevtool/src/config.rs	0.00% <0.00%> (ø)
src/risedevtool/src/service_config.rs	0.00% <0.00%> (ø)
src/risedevtool/src/task.rs	0.00% <ø> (ø)
... and 56 more

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