add aggregation support for date type

[PSeitz]

Add support for date type in aggregations

Histogram

return additionally key_as_string formatted as Rfc3339

{
    "key_as_string" : "2019-01-01T00:00:00Z",
    "key" : 1546300800000000,
    "doc_count" : 1
}

Range

return additionally from_as_string, to_as_string formatted as Rfc3339.
Currently unsupported: pass ranges as formatted text.

{
  "key" : "*-2019-01-01T00:00:00Z",
  "to" : 1546300800000000.0,
  "to_as_string" : "2019-01-01T00:00:00Z",
  "doc_count" : 0
}

[codecov-commenter]

Codecov Report

Merging #1693 (ee8fd25) into main (e758080) will increase coverage by 0.02%.
The diff coverage is 98.05%.

@@            Coverage Diff             @@
##             main    #1693      +/-   ##
==========================================
+ Coverage   94.04%   94.07%   +0.02%     
==========================================
  Files         256      257       +1     
  Lines       49169    49452     +283     
==========================================
+ Hits        46243    46522     +279     
- Misses       2926     2930       +4     

Impacted Files	Coverage Δ
src/fastfield/mod.rs	99.73% <ø> (ø)
src/aggregation/date.rs	69.23% <69.23%> (ø)
src/aggregation/bucket/histogram/histogram.rs	99.55% <98.59%> (-0.07%)	⬇️
src/aggregation/agg_req_with_accessor.rs	95.68% <100.00%> (+1.89%)	⬆️
src/aggregation/agg_result.rs	76.92% <100.00%> (+1.41%)	⬆️
src/aggregation/bucket/range.rs	97.35% <100.00%> (+0.32%)	⬆️
src/aggregation/collector.rs	100.00% <100.00%> (ø)
src/aggregation/intermediate_agg_result.rs	98.43% <100.00%> (+0.13%)	⬆️
src/aggregation/metric/stats.rs	97.21% <100.00%> (ø)
src/aggregation/mod.rs	98.87% <100.00%> (+0.01%)	⬆️
... and 12 more

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