feat: implement own LdpIssuer (replacing Titanium)

[paullatzelsperger]

What this PR changes/adds

This PR adds another, simpler-to-use API for the Issuer part of Titanium. The key aspect is that is uses the EDC expansion/compaction algorithm using the
cached documents.

Why it does that

The Titanium implementation lacks some key aspects, such as adding contexts for SignatureSuites and it is generally cumbersome to use, especially with respect to document caching.

Further notes

• the tests were mostly just copied over, no new tests were added
• the old IssuerTests are kept in place for the time being, but they should eventually get deleted.
• no new functionality was added, it was merely refactored.

Linked Issue(s)

Links to eclipse-edc/IdentityHub#154

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[codecov-commenter]

Codecov Report

Attention: 22 lines in your changes are missing coverage. Please review.

Comparison is base (a62205b) 72.10% compared to head (539a121) 72.06%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
...dc/verifiablecredentials/linkeddata/LdpIssuer.java	59.25%	18 Missing and 4 partials ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3604      +/-   ##
==========================================
- Coverage   72.10%   72.06%   -0.04%     
==========================================
  Files         897      898       +1     
  Lines       17919    17973      +54     
  Branches     1018     1022       +4     
==========================================
+ Hits        12920    12952      +32     
- Misses       4562     4580      +18     
- Partials      437      441       +4     

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[jimmarino]

I'm ok after the doc and formatting comments are addressed