Relax endpoint validation requirements

[drf]

This is a valid Astarte endpoint: /my/2endpoint/stuff

But in 0.11, this does not validate and is rejected when installing the interface. The restriction should instead apply only to the first token.

[rbino]

Note that currently it's possible to create a parametric endpoint with the format /my/%{parametric}/stuff and send /my/2endpoint/stuff path, so not parametric endpoint should be allowed to do that too