Allow remote config layers to be lazy fetched

[jonjohnsonjr]

The partial.ConfigLayer implementation needs to fetch the config blob, but we don't always need to do that (e.g. if we just need the digest).

[codecov-commenter]

Codecov Report

Merging #1634 (70171b3) into main (4a79e94) will decrease coverage by 0.03%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main    #1634      +/-   ##
==========================================
- Coverage   72.30%   72.28%   -0.03%     
==========================================
  Files         119      119              
  Lines        9566     9578      +12     
==========================================
+ Hits         6917     6923       +6     
- Misses       1943     1947       +4     
- Partials      706      708       +2     

Impacted Files	Coverage Δ
pkg/v1/remote/image.go	73.38% <50.00%> (-2.21%)	⬇️

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