Combine Gateway services from different HTTPRoutes for expression router

[randmonkey]

Split from #5801.
The current translator for HTTPRoute translates to traditional routes and expression based routes in separate functions.

The issue is created to track the work for combining gateway services in translating HTTPRoutes in expressions router flavors.

Acceptance Criteria

• When using expressions router flavor, All HTTPRoutes that use identical targetRefs (same service, weight, filters) point to a a single Kong Service