feat: merge package credentials back from oras-credentials-go

[Wwwsylvia]

1. Add package credentials (migrated from oras-credentials-go) and its corresponding description
2. Add new type auth.CredentialFunc
3. Migrate the credentials example from oras.land
4. Change auth.DefaultCache to auth.NewCache() in examples

Resolves: #584

[codecov-commenter]

Codecov Report

Merging #589 (75100f8) into main (92eb05c) will increase coverage by 0.14%.
Report is 1 commits behind head on main.
The diff coverage is 76.25%.

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@@            Coverage Diff             @@
##             main     #589      +/-   ##
==========================================
+ Coverage   74.21%   74.36%   +0.14%     
==========================================
  Files          51       58       +7     
  Lines        4832     5246     +414     
==========================================
+ Hits         3586     3901     +315     
- Misses        914      991      +77     
- Partials      332      354      +22     

Files Changed	Coverage Δ
registry/remote/credentials/trace/trace.go	0.00% <0.00%> (ø)
registry/remote/credentials/native_store_linux.go	80.00% <80.00%> (ø)
...istry/remote/credentials/internal/config/config.go	80.36% <80.36%> (ø)
registry/remote/credentials/store.go	80.95% <80.95%> (ø)
registry/remote/credentials/registry.go	81.25% <81.25%> (ø)
registry/remote/credentials/native_store.go	83.33% <83.33%> (ø)
registry/remote/auth/client.go	73.12% <100.00%> (ø)
registry/remote/credentials/file_store.go	100.00% <100.00%> (ø)

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[shizhMSFT]

LGTM