Extend service interface pattern to decouple service from APIRouter

[critt]

I previously added an interface that GCPService could implement so the sockets handlers and whatever service they use to to process audio data aren't tightly coupled.

There is a similar coupling going on with the APIRouter used for the REST endpoints:

@router.get("/getSupportedLanguages")
async def get_supported_languages():
    return GCPService.get_supported_languages()

Let's do the same thing here:

1. Create an interface, like IRestService or something
2. Have GCPService implement that one too
3. Refactor APIRouter to interact with the interface instead of GCPService