simplify elegant shutdown

[mariomac]

This PR simplifies elegant shutdown by sharing a single channel for all the components that want to listen for a shutdown signal.

Instead of a global variable, ideally this could be achieved by passing a cancellable context to all the running operations but the pipes library is not yet prepared for that (in my to-do list).

[codecov-commenter]

Codecov Report

Merging #190 (10a217c) into main (43ca3a2) will decrease coverage by 0.29%.
The diff coverage is 81.81%.

@@            Coverage Diff             @@
##             main     #190      +/-   ##
==========================================
- Coverage   58.55%   58.25%   -0.30%     
==========================================
  Files          58       58              
  Lines        3373     3349      -24     
==========================================
- Hits         1975     1951      -24     
  Misses       1269     1269              
  Partials      129      129              

Flag	Coverage Δ
unittests	58.25% <81.81%> (-0.30%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/pipeline/extract/aggregate/aggregates.go	90.56% <0.00%> (-0.35%)	⬇️
pkg/pipeline/ingest/ingest_collector.go	63.63% <0.00%> (+1.53%)	⬆️
pkg/pipeline/encode/encode_prom.go	81.69% <100.00%> (-0.25%)	⬇️
pkg/pipeline/ingest/ingest_file.go	47.36% <100.00%> (-1.79%)	⬇️
pkg/pipeline/ingest/ingest_grpc.go	67.74% <100.00%> (-1.96%)	⬇️
pkg/pipeline/ingest/ingest_kafka.go	82.29% <100.00%> (-3.57%)	⬇️
pkg/pipeline/utils/exit.go	100.00% <100.00%> (ø)
pkg/pipeline/write/write_loki.go	65.53% <100.00%> (-0.58%)	⬇️

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