add Indy Credx as credential implementation besides Indy SDK

[TimoGlastra]

Create abstraction in AFJ that allows us to use both the current, Indy SDK, credentials and a new Indy Credx credential implementation

• [ ] https://github.com/animo/aries-mobile-sdk/issues/6
• [ ] https://github.com/animo/aries-mobile-sdk/issues/15
• [ ] Define IndyIssuerService, IndyHolderService, IndyVerifierService interfaces
• [ ] Create IndyIssuerService, IndyHolderService, IndyVerfierService implementations that leverages indy-credx