feat: allow the interface on which the server will listen to be configured (#2015)

# What does this PR do?

It may not always be desirable to listen on all interfaces, which is the
default. As an example, by listening instead only on a loopback
interface, the server cannot be reached except from within the host it
is run on. This PR makes this configurable, through a CLI option, an env
var or an entry on the config file.

## Test Plan

I ran a server with and without the added CLI argument to verify that
the argument is used if provided, but the default is as it was before if
not.

Signed-off-by: Gordon Sim <gsim@redhat.com>

llama_stack/templates/meta-reference-gpu/run-with-safety.yaml

@@ -144,4 +144,3 @@ tool_groups:
   provider_id: rag-runtime
 server:
   port: 8321
-  disable_ipv6: false

llama_stack/templates/meta-reference-gpu/run.yaml

@@ -129,4 +129,3 @@ tool_groups:
   provider_id: rag-runtime
 server:
   port: 8321
-  disable_ipv6: false