refactor(frontend,streaming): separate output pk and internal key in topNExecutor

[lmatz]

What's changed and what's your intention?

As #2130 suggests, the internal key used in TopNExecutor is different from the output primary key of TopNExecutor.

This PR separates these two concepts.

Notice that as previously, the column used in both order by and primary keys will be deduplicated when used as the internal key. Only the backend, not the frontend, is now aware of this optimization.

Checklist

• I have written necessary docs and comments

Refer to a related PR or issue link (optional)

closes #2130
closes #2022

[codecov]

Codecov Report

Merging #2258 (d0c1db2) into main (65a1958) will increase coverage by 0.00%.
The diff coverage is 80.00%.

@@           Coverage Diff           @@
##             main    #2258   +/-   ##
=======================================
  Coverage   71.24%   71.24%           
=======================================
  Files         655      655           
  Lines       83523    83544   +21     
=======================================
+ Hits        59507    59522   +15     
- Misses      24016    24022    +6     

Flag	Coverage Δ
rust	71.24% <80.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
src/stream/src/executor/top_n.rs	0.00% <0.00%> (ø)
src/stream/src/executor/top_n_appendonly.rs	0.00% <0.00%> (ø)
...rc/frontend/src/optimizer/plan_node/stream_topn.rs	52.83% <40.00%> (-17.02%)	⬇️
src/stream/src/executor_v2/top_n.rs	91.03% <95.91%> (+0.14%)	⬆️
src/stream/src/executor_v2/hash_join.rs	81.69% <100.00%> (ø)
src/stream/src/executor_v2/top_n_appendonly.rs	94.46% <100.00%> (+0.03%)	⬆️
...c/executor_v2/managed_state/aggregation/extreme.rs	90.02% <0.00%> (-0.27%)	⬇️

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[fuyufjh]

LGTM