Grow our own `response` package as in LXD

[roosterfish]

Recently we ran into trouble with the dependency to the lxd/response package downstreams have to use when defining their microcluster API handlers. If they the version used for defining the handler diverges from the one used internally by microcluster you might end up with build errors.

To overcome this limitation microcluster should have its own implementation of response which can also be more lightweight as we don't require all the different response types offered by LXD.