Previously all wizard items not of type credential were forced to be last,
but it suffices to force only items having no credential type to be last
(regardless of their type), because only those cannot be taken into account
in the dependency expansion algorithm.

feat: add flag to wizard (dis)allowing it to be started by requestors other than the owning requestor

Add tests for validating circalar dependencies and missing FAQSummary in wizard dependencies. Plus minor refactoring in the implementation.

Improve error messages with dependency chains and extract validateCircularity Method for readability.

This adds support for "issuance wizards" in requestor schemes.

The purpose of an issuance wizard is to guide IRMA app users through a chain of
issuance sessions, for example when the user is registering at a service that
requires the user to disclose attributes coming from multiple issuers. An
issuance wizard will generally mainly consist of credential types, which are
issued in the IRMA app to the user during execution of the wizard, but they can
also end with a disclosure session, or with sending the user to a website.
Issuance wizards can additionally contain text that explains to the user what is
going on.

This also introduces the notion of dependencies of credential types: a
credential type can now depend on one or more other credential types. If
credential type A depends on B and C, and credential type A is included in a
wizard, then the wizard will automatically also include B and C. (This can be
disabled in the wizard if desired, so that all required credential types must be
explicitly included in it.)

feat: revert to earlier irma_configuration structure without issuer_schemes and requestor_schemes subdirectories

The requirement that the directory name in which a scheme is stored equals the scheme name is also dropped.

- All code related to schemes has been moved to schemes.go.
- Code that is specific to a scheme type has been converted to methods on
  the relevant scheme struct, which both implement the new scheme interface.
- Code that is the same for both scheme types (downloading, updating, installing,
  parsing the index, verifying signature, reading timestamp) remain methods on
  irma.Configuration, which invoke methods on the new scheme interface when
  necessary.