feat: Implement merge_sorted for struct

[JakubValtar]

This PR implements merge_sort by struct-typed columns and fixes a bug where the field column names would be empty after merging.

Closes #20986
Closes #13485

import polars as pl

df1 = pl.DataFrame({
    "a": [{"x": 1, "y": 1}, {"x": 1, "y": 3}, {"x": 2, "y": 2}],
})
df2 = pl.DataFrame({
    "a": [{"x": 1, "y": 2}, {"x": 2, "y": 1}, {"x": 2, "y": 3}],
})

print(df1.merge_sorted(df2, "a"))

# shape: (6, 1)
# ┌───────────┐
# │ a         │
# │ ---       │
# │ struct[2] │
# ╞═══════════╡
# │ {1,1}     │
# │ {1,2}     │
# │ {1,3}     │
# │ {2,1}     │
# │ {2,2}     │
# │ {2,3}     │
# └───────────┘

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.76%. Comparing base (0654544) to head (d404486).
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21205      +/-   ##
==========================================
- Coverage   79.88%   79.76%   -0.12%     
==========================================
  Files        1593     1594       +1     
  Lines      227754   228148     +394     
  Branches     2600     2600              
==========================================
+ Hits       181937   181989      +52     
- Misses      45220    45562     +342     
  Partials      597      597              

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[coastalwhite]

Thank you @JakubValtar!

[JakubValtar]

Thanks @coastalwhite for a quick review and answering my questions!