Add CODEOWNERS file to auto-assign reviewers

[erimatnor]

A CODEOWNERS file allows assigning default reviewers for a branch,
obviating the need to add reviewers manually.

[pmwkaa]

seems like a nice feature 👍

[codecov]

Codecov Report

Merging #1646 into master will increase coverage by 0.33%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #1646      +/-   ##
=========================================
+ Coverage   91.56%   91.9%   +0.33%     
=========================================
  Files         144     142       -2     
  Lines       21959   21014     -945     
=========================================
- Hits        20107   19312     -795     
+ Misses       1852    1702     -150

Flag	Coverage Δ
#cron	?
#pr	91.9% <ø> (-0.02%)	⬇️

Impacted Files	Coverage Δ
src/plan_add_hashagg.c	43.33% <0%> (-45.01%)	⬇️
src/telemetry/uuid.c	84.61% <0%> (-15.39%)	⬇️
src/planner_import.c	58.01% <0%> (-12.13%)	⬇️
src/cache_invalidate.c	78.12% <0%> (-2.44%)	⬇️
src/bgw_policy/chunk_stats.c	83.92% <0%> (-1.56%)	⬇️
src/loader/loader.c	91.57% <0%> (-1.25%)	⬇️
src/histogram.c	89.02% <0%> (-1.22%)	⬇️
src/estimate.c	86.17% <0%> (-0.83%)	⬇️
src/telemetry/telemetry.c	79.55% <0%> (-0.71%)	⬇️
src/cache.c	84.82% <0%> (-0.65%)	⬇️
... and 76 more

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