feat: legacy credentials package

[rflechtner]

addresses https://github.com/KILTprotocol/ticket/issues/2156

Adds a package for legacy kilt credential support, which also becomes the new home for transformations between VCs and legacy credentials.

How to test:

Tests included. Doesn't hurt to try exporting a credential with it though.

Checklist:

• I have verified that the code works
• I have verified that the code is easy to understand
  • If not, I have left a well-balanced amount of inline comments
• I have left the code in a better state
• I have documented the changes (where applicable)
  • Either PR or Ticket to update the Docs
  • Link the PR/Ticket here

[ntn-x2]

I assume the Claim.ts and Credential.ts have simply been copy-pasted from the core package? I also did not check the tests in detail yet, also not sure I would be able to provide much feedback on those.

In general, I like a lot the API of the legacy package, with one question about we can make creating a VC from a legacy credential easier for people.

[rflechtner]

I assume the Claim.ts and Credential.ts have simply been copy-pasted from the core package?

Yes indeed, with only minuscule adjustments.

In general, I like a lot the API of the legacy package, with one question about we can make creating a VC from a legacy credential eas_ier_ for people.

This is what is not super easy per se, because it involves an indexer service that we have yet to implement - once we have that we may try to hide this aspect a little better?

[ntn-x2]

LGTM so far!

[ntn-x2]

Not much has changed, and nothing seems to be off.