Add tool to check unmapped fields in Alloy

[marctc]

This utility is based on #1722. It checks that configuration options in Beyla without
the //nolint:undoc are mapped in Alloy. Then it creates a GitHub issue to track those fields.

This will help everytime that we need to update Beyla in Alloy.
It updates this issue every run: #1725

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 284 lines in your changes missing coverage. Please review.

Project coverage is 67.00%. Comparing base (90f6993) to head (d284b0d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
tools/alloy-field-checker/main.go	0.00%	284 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1723      +/-   ##
==========================================
- Coverage   67.87%   67.00%   -0.88%     
==========================================
  Files         213      214       +1     
  Lines       21931    22215     +284     
==========================================
- Hits        14886    14885       -1     
- Misses       6311     6596     +285     
  Partials      734      734              

Flag	Coverage Δ
integration-test	54.43% <ø> (-0.01%)	⬇️
k8s-integration-test	54.03% <ø> (+0.10%)	⬆️
oats-test	34.88% <ø> (ø)
unittests	43.89% <0.00%> (-0.57%)	⬇️

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[grcevski]

LGTM! Very, very cool!