feat(anoncreds): add getCredential(s) methods

[TimoGlastra]

Fixes #1263

[codecov-commenter]

Codecov Report

Merging #1386 (9c41568) into main (4a6b99c) will increase coverage by 0.41%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##             main    #1386      +/-   ##
==========================================
+ Coverage   84.83%   85.24%   +0.41%     
==========================================
  Files         786      786              
  Lines       19371    19383      +12     
  Branches     3151     3151              
==========================================
+ Hits        16433    16523      +90     
+ Misses       2931     2853      -78     
  Partials        7        7              

Impacted Files	Coverage Δ
packages/anoncreds/src/AnonCredsApi.ts	68.71% <0.00%> (-1.73%)	⬇️
...s/anoncreds/src/services/AnonCredsHolderService.ts	100.00% <ø> (ø)
...sdk/src/anoncreds/services/IndySdkHolderService.ts	8.13% <0.00%> (-0.28%)	⬇️
...ncreds-rs/src/services/AnonCredsRsHolderService.ts	92.05% <100.00%> (+0.21%)	⬆️

... and 3 files with indirect coverage changes

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