[http] Current RouteConfigOptions should allow optionally declaring access

[TinaHeiligers]

This issue is to handle the work needed for step 1 in
[http] Prevent access to internal-only APIs when running in serverless

Note: Some (if not most) of the individual issues might need to be handled at the same time. For now, we assume that any work specifically addressing VersionedRouters can be done in parallel.

Adapt the current server-side route declaration

Adapt RouteConfigOptions to:

• Add a new optional access parameter
• This parameter can be set to either public or internal.
• When unspecified, the value is inferred from the route path:
  • If the path starts with /internal, it will default to internal.
  • Otherwise, it will default to public.

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)