Separate modules for the API framework and endpoint implementations

[adamchalmers]

Currently our main src/ directory contains two kinds of modules:

• modules for the API framework (e.g. response or apiclient
• modules for requests/responses from particular Cloudflare APIs (e.g. dns or workerskv)

We should sort these into two different modules (my suggested names: framework and endpoints). That way, most people can ignore the framework directory and only focus on adding their own endpoints.