fix: credential manifest omit optional properties

[HeidiHan0000]

Closes #3335

Summary:

• omitempty tag for mandatory properties were removed
• optional properties (with the omitempty tag) of custom types were changed to pointers so they can be omitted with the omitempty tag
• additional validation checks which were missing and more unit tests

[codecov]

Codecov Report

Merging #3336 (2ac545b) into main (e5f18f2) will increase coverage by 0.00%.
The diff coverage is 95.58%.

❗ Current head 2ac545b differs from pull request most recent head 4a716b1. Consider uploading reports for the commit 4a716b1 to get more accurate results

@@           Coverage Diff           @@
##             main    #3336   +/-   ##
=======================================
  Coverage   88.04%   88.05%           
=======================================
  Files         327      327           
  Lines       45178    45221   +43     
=======================================
+ Hits        39779    39819   +40     
- Misses       3985     3987    +2     
- Partials     1414     1415    +1     

Impacted Files	Coverage Δ
pkg/doc/cm/credentialfulfillment.go	86.36% <ø> (ø)
pkg/doc/cm/credentialmanifest.go	92.52% <94.91%> (-0.07%)	⬇️
pkg/doc/cm/credentialapplication.go	92.80% <100.00%> (+0.11%)	⬆️
pkg/doc/cm/resolve.go	90.96% <100.00%> (ø)

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