feat: simplify humatest, allow sending body slice/map/struct, docs

[danielgtaylor]

This PR:

• Simplifies humatest by removing the custom test context & adapter, instead exposing the context/adapter creation for humachi and piggybacking on that, which also improves the level of dog-fooding in the project.
• Enables the use of a slice/map/struct as input to the test API request functions, which will then automatically add a Content-Type: application/json header and serialize the input as JSON for the request body.
• Documentation updates & examples, which result in nice VSCode popups showing how to use the calls.