feat: adds support for linked attachments

[berendsliedrecht]

Implements 0641.

It allows for attachments outside of the credential to be linked to a credential. This means that it will be way less resource intensive as it does not have to sign the entire attachment, but just the link (a sub 100 char string).

For now a default threshold has been set at 5 megabytes inside the mediator as with bigger images it would just get too cumbersome and as stated in the attachment RFC, messages should remain small. Attachments above 5 megabytes are possible, however they have to be hosted and the link(s) have to be supplied in the linked attachment object.

this will help a lot of mobile agents as after some testing with larger attachments (1mb+) in the Animo react native wallet, it crashed.

Setting of the threshold might be better to do somewhere else, but since the mediator is external it might be not something worth implementing.

[TimoGlastra]

Overall looks good, just some suggestions :)

[codecov-commenter]

Codecov Report

Merging #320 (6a11460) into main (a79e4f4) will increase coverage by 0.13%.
The diff coverage is 94.35%.

@@            Coverage Diff             @@
##             main     #320      +/-   ##
==========================================
+ Coverage   87.86%   87.99%   +0.13%     
==========================================
  Files         224      226       +2     
  Lines        4127     4223      +96     
  Branches      491      520      +29     
==========================================
+ Hits         3626     3716      +90     
- Misses        501      507       +6     

Impacted Files	Coverage Δ
src/decorators/attachment/AttachmentExtension.ts	53.84% <ø> (ø)
src/modules/ledger/services/LedgerService.ts	84.84% <0.00%> (ø)
src/utils/attachment.ts	73.33% <73.33%> (ø)
src/modules/credentials/CredentialUtils.ts	98.07% <91.66%> (-1.93%)	⬇️
src/utils/LinkedAttachment.ts	92.85% <92.85%> (ø)
src/modules/credentials/CredentialsModule.ts	87.17% <100.00%> (+0.51%)	⬆️
...les/credentials/messages/IssueCredentialMessage.ts	96.00% <100.00%> (+0.16%)	⬆️
...les/credentials/messages/OfferCredentialMessage.ts	96.55% <100.00%> (+0.12%)	⬆️
...s/credentials/messages/ProposeCredentialMessage.ts	100.00% <100.00%> (ø)
...s/credentials/messages/RequestCredentialMessage.ts	96.15% <100.00%> (+0.15%)	⬆️
... and 10 more

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