Add support for BLS signatures with BLS keys

[tplooker]

In some cases for instance when a party would like to prove control of a key pair that will be used as the cryptographic binding to say a credential as documented here signatures offer an easy mechanism to do so and BLS signatures as a scheme are a good candidate to solve this problem

[kdenhartog]

interesting I hadn't thought about the BLS signatures being used to solve #37 I'd be interested to hear more about the direction you see with this.

I'd traditionally thought about them for use cases where multiple issuers sign the same credential and the signatures get aggregated together to show that "3 trusted issuers assert my name is Kyle" while also keeping the verifiable presentation small.