Validate openrpc.json in CI

[ValentinTrinque]

Add a document structure validation, in the CI. It's possible to add it as a go test using the official golang marshaller:
https://github.com/open-rpc/meta-schema/blob/master/openrpc_document.go

However, the file doesn't compile because it's missing the definition of Undefined.

In the meantime, we already have tests that (should) ensure the documentation is in sync with the code.

Prototype

package api_test

import (
	vgfs "code.vegaprotocol.io/vega/libs/fs"
	"github.com/open-rpc/meta-schema"
	"github.com/stretchr/testify/require"

	"encoding/json"
	"testing"
)

func TestOpenRPCDocumentIsValid(t *testing.T) {
	rawFileContent, err := vgfs.ReadFile("./openrpc.json")
	require.NoError(t, err)

	document := &openrpc_document.OpenrpcDocument{}
	err = json.Unmarshal(rawFileContent, document)
	require.NoError(t, err)
}