[Feature] There should be a way to enumerate all routes of OpenSearch

[peternied]

Is your feature request related to a problem? Please describe.
As a plugin developer, we've added several APIs and missed supporting our legacy or new route scheme, seeing the routes would give us a way to confirm we've done the right thing.

Additionally as an operator of an OpenSearch cluster it is useful to see the available APIs when I am forgetful or if I want to understand the surface area of the service.

Describe the solution you'd like
Similar to the /_cat/ call there would be a way to inventory all the supported routes of OpenSearch including the installed plugins.

%  curl https://localhost:9200/_cat/routes
|\---/|
| o_o |
 \_^_/
/_cat/allocation [GET]
/_cat/routes [GET]
...
/_plugins/_alerting/monitor/{monitor_id} [GET, POST, PUT, DELETE]
/_plugins/_alerting/monitors/_search [GET, POST]
/_plugins/_alerting/stats [GET]
/_plugins/_alerting/stats/{metric} [GET]

Describe alternatives you've considered
While these registered routes could be displayed output to the log, that doesn't seem as useful for a more adhoc check, but that would be useful for system administrators if they want to make sure they know the capabilities of the cluster.

Additional context

• Brainchild from Introduce new API _plugins/_security/ssl/certs security#1841

[dblock]

The RESTful API should be generated from spec (#3090), making this easy. The service would just return the spec.