Adding Spiel backend support

**Is your feature request related to a problem? Please describe.**

From @eeejay :

It would be useful to be able to start speech syntheses from e.g. flatpak/snap containers to isolate them. The Spiel approach allows to easily ship syntheses this way, and connect to them through dbus.

**Describe the solution you'd like**

We could add to the speechd core support for Spiel, in addition to the existing modules support. Basically, a new implementation alongside `output.c`.

Start by reading the speechd core and output.c to understand how existing modules are supported. Then determine the required Spiel and D-Bus integration boundaries and how it should coexist with the current module support. Done means the core can start speech syntheses through Spiel, with coverage for the new behavior.

Written by the indexing model from the issue text.