Unexport client attributes

[EmmEff]

This PR unexports client attributes that do not need to be exported.

[wobito]

Singularity build fails with this PR seems to use the BaseURL attribute

../internal/app/singularity/push.go:118:45: libraryClient.BaseURL undefined (type *"github.com/sylabs/scs-library-client/client".Client has no field or method BaseURL) 

[EmmEff]

Good catch! Singularity will need a patch to be able to use v2 of scs-library-client, which I will create once the first v2 release has been tagged.

[codecov-commenter]

Codecov Report

Merging #167 (06a0ab5) into main (7ba7416) will increase coverage by 0.03%.
The diff coverage is 46.06%.

❗ Current head 06a0ab5 differs from pull request most recent head 7a70528. Consider uploading reports for the commit 7a70528 to get more accurate results

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@@ Coverage Diff @@ ## main #167 +/- ## ========================================== + Coverage 34.59% 34.62% +0.03%  ========================================== Files 13 13 Lines 2006 2007 +1 ========================================== + Hits 694 695 +1  Misses 1235 1235 Partials 77 77 

Impacted Files	Coverage Δ
client/push.go	10.87% <9.75%> (ø)
client/oci.go	5.67% <42.85%> (ø)
client/version.go	60.00% <50.00%> (ø)
client/pull.go	50.87% <66.66%> (ø)
client/api.go	60.49% <83.33%> (ø)
client/client.go	94.54% <100.00%> (+0.10%)	⬆️
client/downloader.go	75.29% <100.00%> (ø)
client/restclient.go	75.80% <100.00%> (ø)

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