Callback describes behaviours required for notifying a client application of
issuance or presentation flow status.
The Client trait is used by implementers to provide Client metadata to the
library.
Context is implemented by every endpoint to set up a context for each
request.
The Holder trait specifies how the library expects user information to be
provided by implementers.
The Issuer trait is used by implementers to provide Credential Issuer
metadata.
The Issuer trait is used by implementers to provide Authorization Server
metadata.
Signer is used by implementers to provide signing functionality for
Verifiable Credential issuance and Verifiable Presentation submissions.
StateManager
is used to store and manage server state.