feat: support more backend protocols for ext & int LBs

What

Support more backend protocols (HTTP, HTTPS and HTTP2).

Why

Currently AI gateway is the only service using HTTPS for the backend protocol, which Runway does not currently support (prior to this MR). Runway currently supports HTTP or HTTP2.

With the gRPC changes in Runwayctl and due to a bug in Google's lb-http Terraform module, the Terraform plan wants to set the backend protocol to HTTP, and we're not confident it won't break the service. Instead, we're proposing the changes in this MR so we can keep the backend protocol as HTTPS until we figure out if we can switch to HTTP.