osbuild: auto-generate socket addresses for APIs

Rely on the ability of `BaseAPI` to auto-generate socket addresses
when no one was provided. The `BuildRoot` does not rely on the
sockets being created in the `BuildRoot.api` directory anymore and
will instead bind-mount each individual socket address to the well
known location via the `BaseAPI.endpoint` identifier.
Convert all API providers to take the `socket_address` as an
optional keyword argument.

osbuild/sources.py

@@ -8,7 +8,7 @@ class SourcesServer(api.BaseAPI):
 
     endpoint = "sources"
 
-    def __init__(self, socket_address, libdir, options, cache, output):
+    def __init__(self, libdir, options, cache, output, *, socket_address=None):
         super().__init__(socket_address)
         self.libdir = libdir
         self.cache = cache