core (main): Let plugins have several handlers + various improvements: `get_handler` can now return several handlers, which is notably useful for component with several included services. Various improvements such move to `async` method, docstring/type hints update, and slight improvment of SatXMPPComponent. rel 453

this certificates are used to enable TLS during tests. Those must be the same as in
backend_e2e (they are generated there)