Run-time plugin router definition

[davidbrochart]

Problem

Currently the API paths have to be defined at import-time, because FPS registers all the plugins through entry points and checks for path conflicts, etc. But we might want API paths to be configurable at run-time.

Proposed Solution

Could it be possible to defer the registration of routers, so that they have been configured before they are registered?
Maybe plugins could have initialize() and get_routers() callback hooks, where initialize passes configuration to a plugin before get_routers gets its routers back.

Additional context

For instance, Voila allows to configure the path to statically served directories, but if we want to implement that using FPS, we cannot use mounting points, we need to hack by defining an endpoint with a generic path, and implement the dynamic behavior inside the handler.