Add links to child paths for /actuator/gateway endpoint

[csterwa]

Problem

When there are child paths for actuator endpoints it is useful to have a list of those endpoints on the parent or at least make them accessible. Currently, the /actuator/gateway endpoint does not have output providing an overview of what is available on the child actuator endpoints. Adding a / controller endpoint for the actuator here could resolve this for users.

Proposed Solution

Add output to the /actuator/gateway endpoints with links to the child actuator endpoints:

• /actuator/gateway/refresh
• /actuator/gateway/routes
• /actuator/gateway/routes/{id}
• /actuator/gateway/routes/{id}/combinedfilters
• /actuator/gateway/routefilters
• /actuator/gateway/routepredicates

Current Situation

Currently, this is only found via the docs and is not navigable using the /actuator/gateway actuator endpoint.