Allow both Tag and Digest (Reference) for write

[afbjorklund]

Having a RepoTags entry is optional, it is possible
to write without tags using digests - making it "null"

Added convenience functions and new "Ref" variants,
since it is not possible to cast arrays of interfaces.

[codecov-io]

Codecov Report

Merging #404 into master will decrease coverage by 0.08%.
The diff coverage is 68%.

@@            Coverage Diff             @@
##           master     #404      +/-   ##
==========================================
- Coverage   52.36%   52.27%   -0.09%     
==========================================
  Files          78       78              
  Lines        3739     3753      +14     
==========================================
+ Hits         1958     1962       +4     
- Misses       1486     1496      +10     
  Partials      295      295

Impacted Files	Coverage Δ
pkg/v1/tarball/write.go	60.91% <68%> (-6.21%)	⬇️

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