Support for pushing SIF images directly to OCI registry

[EmmEff]

As above... follows work for OCI direct downloads in #150

#150 must be merged first

[codecov-commenter]

Codecov Report

Merging #151 (56cb352) into main (915e8a8) will decrease coverage by 6.19%.
The diff coverage is 15.98%.

❗ Current head 56cb352 differs from pull request most recent head 3f146c2. Consider uploading reports for the commit 3f146c2 to get more accurate results

@@            Coverage Diff             @@
##             main     #151      +/-   ##
==========================================
- Coverage   41.25%   35.06%   -6.20%     
==========================================
  Files          13       13              
  Lines        1738     2042     +304     
==========================================
- Hits          717      716       -1     
- Misses        938     1243     +305     
  Partials       83       83              

Impacted Files	Coverage Δ
client/push.go	10.87% <0.00%> (-0.19%)	⬇️
client/oci.go	5.38% <5.38%> (-4.56%)	⬇️
client/client.go	94.54% <25.00%> (ø)
client/pull.go	53.94% <67.44%> (ø)
client/downloader.go	75.00% <75.00%> (ø)

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[tri-adam]

One small suggestion, otherwise LGTM. Thank you.

[DrDaveD]

Does this and #151 have an impact on the library API standard? Does the standard need an extension?

[dtrudg]

With respect to the API standard, please see singularityhub/library-api#5