Dynamic client registration should allow for a software_id to be configured and used

[travisspencer]

While the op_test tool is for testing OIDC dynamic client registration, some OPs also support the OAuth dynamic client registration protocol. This spec defines some additional inputs that the registering client must send to the OP/AS. It would make the op_test tool more useful if some/all of these were possible to configure. In particular, the software_id defined in that spec is quite useful in a number of use cases. To support the testing of these, the op_test tool should allow for a software_id to be configured:

If this is configured, the test tool should include this in the body of the client registration request as JSON (per RFC 7591 and section 3.1 of the corresponding OIDC spec).

[selfissued]

Like #59 this would be useful functionality but can't be required for certification.