feat: compare owner and signer of credential on signature verification

[rflechtner]

fixes KILTProtocol/ticket#1773

Checking whether the signer of a credential is (related to) the owner has is so far not happening as part of verifyPresentation.
This adds this feature to verifyDidSignature, which is called by verifyPresentation.

Note: This cherry-picks a commit (01bf6fd) originally included with #675.

How to test:

Unit tests related to that issue did not exist yet. I added these.

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)

[ntn-x2]

Can we maybe move the logic inside the isSameSubject() function? And modify it to perhaps take a checkType flag, which fails if the subject is the same but the DID is of a different type? Flag false would return true if the subject is the same regardless of the type.

Also I don't think we need directionality in the type check, right? If one is light and one is full, it is always an error, so that could simplify the verification logic a bit?

Like signing something with a light DID when a full DID exist is an error, and signing a DID with a full DID that does not exist is also an error.

[rflechtner]

Can we maybe move the logic inside the isSameSubject() function? And modify it to perhaps take a checkType flag, which fails if the subject is the same but the DID is of a different type? Flag false would return true if the subject is the same regardless of the type.

Also I don't think we need directionality in the type check, right? If one is light and one is full, it is always an error, so that could simplify the verification logic a bit?

Like signing something with a light DID when a full DID exist is an error, and signing a DID with a full DID that does not exist is also an error.

I was looking into how this interacts with the isSameSubject function but the check we do here seems really specific to credentials. We have a situation here where we want to allow creds owned by a light did to be signed by the corresponding full did, but not the other way around. That implies directionality, which seems out of scope for isSameSubject.
In fact this check is so specific to this use case, that I even decided against having it as a feature on did signature verification more generally.

[rflechtner]

I ended up pulling the logic into verifyDidSignature because #675 requires something similar. What do you think, @ntn-x2 ?

[ntn-x2]

I like it!