Add tarball.WithMediaType to specify layer media type

[imjasonh]

cc @ekcasey @samj1912

[codecov-commenter]

Codecov Report

Merging #1286 (9a2cce1) into main (a44adc3) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1286      +/-   ##
==========================================
+ Coverage   74.01%   74.03%   +0.01%     
==========================================
  Files         112      112              
  Lines        8374     8379       +5     
==========================================
+ Hits         6198     6203       +5     
  Misses       1573     1573              
  Partials      603      603              

Impacted Files	Coverage Δ
pkg/v1/tarball/layer.go	70.55% <100.00%> (+0.93%)	⬆️

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