Move Jwt functionality from credentials package to dids or crypto package

1. Currently the Jwt functionality lives in the credentials package. This requires installing the `credentials` just to use Jwt creation, signing or verification. To fix we should move Jwt to the DIDs package.

2. Jwt has a hard dependency on `BearerDid` which is an abstraction from the DIDs. If we wanted Jwt to live in the `crypto` package (rather than the DIDs package) then we must remove Jwt's hard dependency on BearerDid. This probably means passing in the signer separately as a parameter to Jwt functions.