[Request] Parametrized signer for IssuerSignedBuilder

[chumbert]

Hello,

as a user of the library I'd like to be able to plug-in my own signing logic within the IssuerSignedBuilder.

The builder offers a lot of convenience but due to the encapsulated sign that requires the private key to be present in-memory, we can't use it with a KMS/HSM that handles the signing operation itself.

As a suggestion, I would add a signer interface designed to be optionally provided when creating an IssuerSignedBuilder and used to replace the private sign method.

Please let me know if these changes are compatible with your vision of the library.

Thanks in advance and best regards.

Update: here's a proposal - #14

[TakahikoKawasaki]

@chumbert
I added the setIssuerAuthSigner(SigStructureSigner) method to the IssuerSignedBuilder class to support the use of a custom signer, and released a new version (1.20) of this library.