Expose configs and files as API in code

[IljaN]

For the "embedding as library use-case" the consumer might want to provide a custom-configuration format (memory,vfs). Currently there is no way to embed lico without having an identifier-registration.yml somewhere on the file-system.

To solve this the configuration should be exposed as map or struct internally.

Also other supporting files like certs and secrets should be passable as an io.Reader or byte[].

Related: Kopano-dev/konnect#24