Project structure: remove nimbus-jose dependency from jwt-spi

[ndr-brt]

@ndr-brt In this case, it may be onerous to wrap and unwrap Nimbus classes. Are there a lot of classes that need to be wrapped?

In the spi we are using three nimbus-jose classes:

• JWSHeader
• JWTClaimsSet
• SignedJWT

the first two of them are actually used paired on the JwtDecorator.decorate method so they can be wrapped by a single class.
I think this is doable, maybe we can address this to another issue for M7

Originally posted by @ndr-brt in #1867 (comment)