deprecate: deprecate all existing APIs

[Wwwsylvia]

1. Deprecate all existing APIs as they have been moved to oras-go
2. Update README accordingly

Resolve: #95

[codecov-commenter]

Codecov Report

Merging #96 (aafd9c7) into main (8b76a43) will decrease coverage by 12.59%.
The diff coverage is 64.70%.

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@@             Coverage Diff             @@
##             main      #96       +/-   ##
===========================================
- Coverage   76.86%   64.28%   -12.59%     
===========================================
  Files           8        6        -2     
  Lines         428       28      -400     
===========================================
- Hits          329       18      -311     
+ Misses         77       10       -67     
+ Partials       22        0       -22     

Files Changed	Coverage Δ
native_store.go	0.00% <0.00%> (-83.34%)	⬇️
trace/trace.go	0.00% <0.00%> (ø)
registry.go	80.00% <80.00%> (-1.25%)	⬇️
file_store.go	100.00% <100.00%> (ø)
memory_store.go	100.00% <100.00%> (ø)
store.go	100.00% <100.00%> (+19.04%)	⬆️

[shizhMSFT]

LGTM