add mock server from consent

[taylordowns2000]

This brings the Consent BB's "mock" application to our template folder. The idea is that each BB should be able to stand up a mock server which replies to HTTP requests according to their own OpenAPI specification. This needs generification and testing, either before or after merging depending on whether we prioritize "getting it out there" or "getting it right"

[rachellawson]

Can we get some level of “genericisation” here? Make it clear that this is a template mock api rather than one for consent?

[rachellawson]

I like it, though for use in the template, we probably need to make it clear it is a generic template rather than something specific for consent. Simply to avoid having to explain that every time.

[benjaoming]

I'm doing some changes in bb-consent, I've added a layer that reproduces static JSON responses from files. I'll probably add more.

It makes sense to me that this PR is merged with @rachellawson's adjustments, but I think we should definitely also find a good process for "backporting" changes from building blocks.