Add configurable stub server

[wimspaargaren]

Add a configurable stub server.

Configuring

curl --location 'localhost:8080/configure' \
--header 'Content-Type: application/json' \
--data '{
    "path": "/foo",
    "response": {"foo": "bar"}
}'

Hit a configured endpoint

curl --location 'localhost:8080/foo'

[sbp-bvanb]

Could you extend the README by adding an example you have provided in the description of this PR?

[wimspaargaren]

Done. Also simplified the docker file. Now the only thing that remains is adjusting the docker GH action to receive different build args.

[markvl-sbp]

I think the Dockerfile can be improved slightly, but nothing to keep this PR from being merged as-is.

[kasbuunk]

I don't have time for detailed look, but some comments nonetheless. If I had more time I'd take a close look at that any in Response and map value in endpoints to see how it's used and if we can make a non-empty interface from that.