jfix-armeria-aggregating-profiler: being able to configure separate thread pool for profiling

[TimurKasatkin]

At the moment profiling is performed at the same thread http requests are executed.
It can be checked by launching tests of jfix-armeria-aggregating-profiler module:

13:28:27.743 [armeria-common-worker-epoll-2-5] ru.fix.armeria.aggregating.profiler.ProfiledHttpClient(ProfiledHttpClient.kt:161)
DEBUG Request log = {req={startTime=2020-12-14T10:28:26.721Z(1607941706721663), length=0B, duration=20293µs(20293764ns), scheme=none+h2c, name=GET, headers=[:method=GET, :path=/ok-delayed/1000, :scheme=http, :authority=localhost:43049]}, res={startTime=2020-12-14T10:28:27.742Z(1607941707742995), length=6B, duration=268µs(268143ns), totalDuration=1021ms(1021601241ns), headers=[:status=200, content-type=text/plain; charset=utf-8, content-length=6, server=Armeria/1.2.0, date=Mon, 14 Dec 2020 10:28:27 GMT]}}; connections timings = {connectionAcquisitionStartTime=2020-12-14T10:28:26.722Z(1607941706722384), connectionAcquisitionDuration=16956µs(16956619ns), dnsResolutionStartTime=2020-12-14T10:28:26.722Z(1607941706722384), dnsResolutionDuration=257µs(257582ns), socketConnectStartTime=2020-12-14T10:28:26.724Z(1607941706724678), socketConnectDuration=14634µs(14634974ns)}

It decreases performance of http requests processing since I/O worker does some additional work (in this case, profiling).

###Proposal

Make it possible to configure thread pool (java.util.concurrent.ExecutorService), used for requests profiling.

[TimurKasatkin]

UPD after jfix-armeria-micrometer creation, Micrometer is a strategic profiler for modern systems. Thus, priority of all issues, related to integration with https://github.com/ru-fix/aggregating-profiler, is decreased.