Enforce copyright check in CI

[marcalff]

Enforce copyright check in CI.

Changes

Added:

• a Copyright check in CI,
• a tools/check_copyright.sh script,
• file .copyright-ignored to allow for copyright exceptions.

Checklist:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed (no changes)

[marcalff]

With the initial commit, the Copyright CI fails, as expected, because all files do not have a proper copyright.

This is an opportunity to check the logs in the failed CI.

After PR #1963 is merged, the list of issues should shrink, and list only remaining items.

[codecov]

Codecov Report

Merging #1965 (01fc8b1) into main (1d18614) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1965   +/-   ##
=======================================
  Coverage   87.12%   87.12%           
=======================================
  Files         165      165           
  Lines        4596     4596           
=======================================
  Hits         4004     4004           
  Misses        592      592           

Impacted Files	Coverage Δ
sdk/src/trace/samplers/trace_id_ratio.cc	100.00% <ø> (ø)

[marcalff]

Ready for review.

[lalitb]

LGTM. nicely done, thanks.