Add missing envoy-gateway conformance function

[heyste]

Fixes: #36

run::envoy-gateway::conformance() {
  echo "run::envoy-gateway::conformance"
  run::envoy-gateway::gateway-api-conformance
}

run::envoy-gateway::gateway-api-conformance

• checks out the gateway-api repo
• switches to the tag release of GATEWAY_API_VERSION
• runs conformance tests ref: https://github.com/kubernetes-sigs/gateway-api/blob/main/conformance/conformance_test.go