router: add pluggable networking providers

[John Howard (howardjohn)]

This PR extends the router to support pluggable network providers rather than a hardcoded Envoy. Envoy remains the default with no changes.

As we build out the feature set of substrate, we will have an expanded set of requirements on networking. For example, #15 (comment) expresses a need to inject credentials and a desire to be able to plug in the proxy of your choice. IMO this is very important: there are a number of projects coming up in this space specializing in the needs of agentic traffic that make sense to plug in here.

• Tests pass
• Appropriate changes to documentation are included in the PR

[Bowei Du (bowei)]

Question: if the AgentGateway doesn't use XDS, you might get broken if some logic gets pushed into the XDS configuration? The XDS is minimal right now, but will highly likely change.

That being said, Envoy should be an implementation detail and I think it makes sense to make this pluggable.

Also, it would be good to use package boundaries to delineate the providers if possible.

[John Howard (howardjohn)]

Question: if the AgentGateway doesn't use XDS, you might get broken if some logic gets pushed into the XDS configuration? The XDS is minimal right now, but will highly likely change.

One note on this - agentgateway does support dynamic configuration via the XDS gRPC APIs it just has different types sent over that service (so cannot use the existing xDS server which sends Envoy-specific Listeners/Clusters/etc).
I kept it as a static file today since there is nothing dynamic but if we need to we can easily move it over to being dynamic.