External Coprocessor: Support for connections to be made with h2c (http2 cleartext/prior knowledge) and/or Unix Domain Sockets

[ShaunPhillips]

Is your feature request related to a problem? Please describe. Router has already implemented support for h2c for subgraph requests: https://github.com/apollographql/router/pull/3852 This request is to provide the same functionality to enable configuring the External Coprocessor plugin to use h2c when making requests to a remote External Coprocessor endpoint. This would be when the endpoint service is deployed as a standalone application vs. as a sidecar container in the same Kubernetes pod.

In addition, support calling the External Coprocessor over a Unix domain socket when the endpoint is deployed as a sidecar container. https://github.com/apollographql/router/pull/4757

Describe the solution you'd like Expose support for h2c and Unix domain socket configuration options for the External Coprocessor plugin via the router.yaml file.