REST DestinationSpec for K8S Gateway API

[DuncanDoyle]

Design and implement the a DestinationSpec API for Rest Function Upstreams for K8S Gateway API.

Context

See als the parent issue for the context and motivation: https://github.com/solo-io/gloo/issues/9712

In our current implementation to support function parameters for Function Upstreams in K8S Gateway API, we use an ExtensionRef to a virtual Parameter resource to pass in, for example, the logical name of the Lambda function to call. See https://github.com/solo-io/solo-projects/issues/6160#issuecomment-2161397367, which contains the following example:

backendRefs:
    - name: lambda
      namespace: gloo-system
      group: gloo.solo.io
      kind: Upstream
      filters:
        - type: ExtensionRef
          extensionRef:
            group: "gloo.solo.io"
            kind: Parameter
            name: echo

Since this is an unnamed parameter, we only support on Function Upstream parameter to be specified. There are however destinationSpec in the Gloo Edge API that support multiple parameters: https://docs.solo.io/gloo-edge/latest/reference/api/github.com/solo-io/gloo/projects/gloo/api/v1/options.proto.sk/#destinationspec

The REST destinationSpec for example supports fields like functionName, parameters and responseTransformation: https://docs.solo.io/gloo-edge/latest/reference/api/github.com/solo-io/gloo/projects/gloo/api/v1/options/rest/rest.proto.sk/#destinationspec

With the current K8S Gateway API implementation, we can't/don't support these additional parameters.

Requirement

Design and implement an API for the K8S Gateway API that provides support for DestinationSpecs with multiple parameters for REST Upstreams.