switch to Aggregation without serde_untagged

[PSeitz]

• refactor result handling
• remove Internal stuff
• merge different accessors
• switch to Aggregation without serde_untagged by manually flattening the enum

Closes quickwit-oss/quickwit#1485

[codecov-commenter]

Codecov Report

Merging #2003 (e841842) into main (c1defdd) will decrease coverage by 0.10%.
The diff coverage is 94.30%.

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@@            Coverage Diff             @@
##             main    #2003      +/-   ##
==========================================
- Coverage   94.44%   94.35%   -0.10%     
==========================================
  Files         320      320              
  Lines       59231    59001     -230     
==========================================
- Hits        55943    55669     -274     
- Misses       3288     3332      +44     

Impacted Files	Coverage Δ
src/aggregation/agg_result.rs	76.92% <ø> (-1.91%)	⬇️
src/aggregation/bucket/mod.rs	96.15% <ø> (ø)
src/aggregation/metric/mod.rs	94.54% <ø> (ø)
src/aggregation/intermediate_agg_result.rs	95.32% <83.96%> (-2.00%)	⬇️
src/aggregation/bucket/term_agg.rs	97.95% <85.18%> (-0.78%)	⬇️
src/aggregation/agg_req.rs	94.11% <86.51%> (+1.03%)	⬆️
src/aggregation/segment_agg_result.rs	93.18% <94.93%> (-2.75%)	⬇️
src/aggregation/metric/percentiles.rs	89.26% <96.29%> (-2.03%)	⬇️
src/aggregation/metric/stats.rs	92.51% <97.82%> (-5.15%)	⬇️
src/aggregation/agg_bench.rs	100.00% <100.00%> (ø)
... and 7 more

... and 2 files with indirect coverage changes

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