build: add license checker and missing license headers

[shizhMSFT]

What this PR does / why we need it

Important

The licenses of dependencies are not checked since SkyWalking-Eyes does not support dotnet projects.

Add a license checker and add missing license headers.

Which issue(s) this PR resolves / fixes

Resolves #61

Please check the following list

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (9650855) 73.07% compared to head (45a0fa2) 73.07%.

Files	Patch %	Lines
src/OrasProject.Oras/Remote/Registry.cs	50.00%	1 Missing ⚠️

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Additional details and impacted files

@@           Coverage Diff           @@
##             main      #86   +/-   ##
=======================================
  Coverage   73.07%   73.07%           
=======================================
  Files          31       31           
  Lines        1040     1040           
  Branches       91       91           
=======================================
  Hits          760      760           
  Misses        228      228           
  Partials       52       52           

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[Wwwsylvia]

LGTM