feat: support warning in remote targets

[qweeah]

What this PR does / why we need it:
Fixes #898

[codecov-commenter]

Codecov Report

Merging #1057 (9a9b387) into main (742b044) will decrease coverage by 0.02%.
The diff coverage is 89.70%.

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@@            Coverage Diff             @@
##             main    #1057      +/-   ##
==========================================
- Coverage   80.00%   79.99%   -0.02%     
==========================================
  Files          56       56              
  Lines        2896     2914      +18     
==========================================
+ Hits         2317     2331      +14     
- Misses        402      405       +3     
- Partials      177      178       +1     

Files Changed	Coverage Δ
cmd/oras/internal/option/remote.go	77.38% <76.00%> (-0.21%)	⬇️
cmd/oras/root/discover.go	84.72% <85.71%> (ø)
cmd/oras/internal/option/target.go	84.69% <100.00%> (+0.31%)	⬆️
cmd/oras/root/attach.go	81.89% <100.00%> (ø)
cmd/oras/root/blob/delete.go	82.60% <100.00%> (ø)
cmd/oras/root/blob/fetch.go	68.26% <100.00%> (ø)
cmd/oras/root/blob/push.go	83.69% <100.00%> (ø)
cmd/oras/root/cp.go	86.56% <100.00%> (ø)
cmd/oras/root/login.go	72.22% <100.00%> (ø)
cmd/oras/root/manifest/delete.go	82.19% <100.00%> (ø)
... and 8 more

[qweeah]

@Wwwsylvia test fixed, can you help review again.

[Wwwsylvia]

LGTM, but I am not a maintainer

[shizhMSFT]

LGTM

[Wwwsylvia]

Should ImageSpec.PackType be changed to ImageSpec.PackVersion?

[TerryHowe]

/lgtm

[TerryHowe]

Hmm, @Wwwsylvia has a comment

[qweeah]

Should ImageSpec.PackType be changed to ImageSpec.PackVersion?

Good catch. This is not related to change here, I will do in another PR.