refactor(rust): Use HashKeys abstraction

[orlp]

This will be used in both the join / reduce interfaces to abstract over a key dataframe which may or may not be pre-hashed.

[codecov]

Codecov Report

Attention: Patch coverage is 1.08696% with 91 lines in your changes missing coverage. Please review.

Project coverage is 79.53%. Comparing base (8cb7839) to head (ff2bbe6).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-expr/src/hash_keys.rs	0.00%	76 Missing ⚠️
crates/polars-expr/src/groups/row_encoded.rs	0.00%	8 Missing ⚠️
crates/polars-stream/src/nodes/group_by.rs	0.00%	5 Missing ⚠️
crates/polars-expr/src/groups/mod.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19785      +/-   ##
==========================================
- Coverage   79.55%   79.53%   -0.02%     
==========================================
  Files        1544     1546       +2     
  Lines      213240   213498     +258     
  Branches     2441     2442       +1     
==========================================
+ Hits       169643   169809     +166     
- Misses      43048    43139      +91     
- Partials      549      550       +1     

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