Unify the Node.js API for providers

[lfades]

Currently we have API methods that work under /api/bigcommerce/* that applies for all providers. The API used for the Node.js side of the commerce demo should be unified in a similar way of the agnostic provider we currently have for the UI hooks.

[Vandivier]

@lfades I support this but let's also think one step higher in abstraction: There are some API that go beyond providers and would be used for any back end. For example, isAllowedMethod.

ref: vercel/next.js#25599

As a side note, I find the higher-order hook implementation like swell/.../use-signup.tsx to be really hard to reason about. Just a note that 'unify the API' doesn't need to be done with a compositional approach. isAllowedMethod, for example, could simply be pasted in framework/utils and the other files could directly import it without composition.

[lfades]

@Vandivier isAllowedMethod was moved to be inside commerce so that's already the case, on the note about compositional approach that's a requirement because providers don't match each other's features, each provider adds what they can do