Add note about the revocation mechanisms in the second edition of 18013-5

[martijnharing]

Resolves #109

[javereec]

While I understand the need for mentioning of a status mechanism, I don't think the current text adds much in terms of clarity.

[Sakurann]

WG discussion:

• ISO specification has two revocation mechanisms - would it make sense to limit it to one of those in HAIP? would that increase interoperability
• we could say similar to what we say on sd-jwt: issuer chooses one option, wallet and verifier must support both?

[Sakurann]

wg discussion: agreed to proceed with option 2.

• option 1: for mdocs mandate both bitlist and identifier for HAIP RPs: align mdoc with sd-jwt

• option 2: for both mdoc and sd-jwt, issuer can pick one of the mechanisms, do not mandate anything either for RPs.

  • update sd-jwt vc section: will do in Reword SD-JWT credential validity text + remove requirement for verifiers/wallets to support status list #276

• two mechanisms in ISO have the same structure, the main difference is whether it is a bitlist or an identifier, which makes it "effectively the same" for the RPs.

  • update this PR with a bit more explanation of the above

• in 18013-5, support for the revocation check is optional for the verifiers, because they can be offline. support by the wallet does not make much sense.

• sd-jwt says "It is at the discretion of the Issuer whether to use exp claim and/or a status claim to express the validity period of an SD-JWT VC. The Wallet and the Verifier MUST support both mechanisms."