perf: rewrite field:* to _exists_:field

[dkharms]

Description

We can rewrite queries like trace_id:* to _exists_:trace_id in regular search requests. The latter query is significantly lighter and significantly easier to compute because we have special _exists_ index type.

This way we do not need to build huge tree of iterators over all possible token values. The following benchmark proves it -- trace_id is a high-cardinal field while k8s_pod is not:

Query	Type	mean (ms)			stddev (ms)			p(50) (ms)			p(95) (ms)			p(99) (ms)			iterations			total
		base	comp	diff	base	comp	diff	base	comp	diff	base	comp	diff	base	comp	diff	base	comp	diff	base	comp	diff
k8s_pod:*	hot	14.79	1.33	-91.02%	1.69	0.18	-89.12%	14.23	1.34	-90.56%	18.74	1.60	-91.44%	46.66	31.86	-31.72%	47	47	0.00%	1019829	1019792	-0.00%
	cold	100.21	84.49	-15.69%	4.85	9.72	+100.30%	99.33	82.52	-16.92%	108.22	106.53	-1.57%	109.86	121.06	+10.19%	27	27	0.00%	1019735	1019292	-0.04%
trace_id:*	hot	211.08	1.38	-99.35%	4.17	0.14	-96.64%	210.42	1.39	-99.34%	224.59	1.63	-99.27%	256.51	31.53	-87.71%	46	47	+2.17%	1019898	1019556	-0.03%
	cold	336.39	77.91	-76.84%	13.45	3.35	-75.07%	333.31	77.47	-76.76%	360.01	92.68	-74.26%	373.03	104.51	-71.98%	23	28	+21.74%	1019355	1019146	-0.02%

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[github-actions]

🔴 Performance Degradation

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Name	Previous	Current	Ratio	Verdict
MutexListAppend-4	a73114	4a34dc
	194.53 MB/s	174.79 MB/s	0.90	🔴

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.30%. Comparing base (a73114c) to head (22080be).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #502      +/-   ##
==========================================
- Coverage   71.33%   71.30%   -0.03%     
==========================================
  Files         233      233              
  Lines       18999    19012      +13     
==========================================
+ Hits        13552    13557       +5     
- Misses       4419     4425       +6     
- Partials     1028     1030       +2     

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