refactor!: add read-only interfaces

[Wwwsylvia]

Add read-only interfaces to support #282

Part of #272
BREAKING CHANGE:

• Change the parameter type of src for oras.CopyGraph function
• Change the parameter type of src for oras.Copy function
• Change the parameter type of src for oras.CopyOptions.MapRoot
• Change the parameter type of src for oras.ExtendedCopyGraph function
• Change the parameter type of src for oras.ExtendedCopy function
• Change the parameter type of src for oras.ExtendedCopyGraphOptions.FindPredecessors
• Change the parameter type of target for oras.Resolve function

Signed-off-by: Sylvia Lei lixlei@microsoft.com

[codecov-commenter]

Codecov Report

Merging #283 (ac89ad4) into main (98e72a2) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #283   +/-   ##
=======================================
  Coverage   70.77%   70.77%           
=======================================
  Files          36       36           
  Lines        3357     3357           
=======================================
  Hits         2376     2376           
  Misses        734      734           
  Partials      247      247           

Impacted Files	Coverage Δ
content/graph.go	0.00% <ø> (ø)
content/storage.go	0.00% <ø> (ø)
internal/registryutil/proxy.go	71.42% <ø> (ø)
content.go	56.25% <100.00%> (ø)
copy.go	60.78% <100.00%> (ø)
extendedcopy.go	68.42% <100.00%> (ø)
internal/cas/proxy.go	78.94% <100.00%> (ø)

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[shizhMSFT]

LGTM