Split BlsSignerFactory.getSigner into direct Fr input and arbitrary seed

[jacque006]

Problem

Per #28 (comment) , BlsSignerFactory.getSigner is not explicit in whether it will hash or directly set the provided secret. This could create compatibility issues with other BLS libs and does not give the consumer a way to know how their secret will be used in signing.

Solution

Split BlsSignerFactory.getSigner into two factory methods:

• BlsSignerFactory.getSigner, which will directly set the provided Fr and throw if it out of range.
• BlsSignerFactory.getSignerHashed, which will hash the provided secret/seed.