Move Jwt functionality from credentials package to dids or crypto package

[shamilovtim]

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.