[http] Versioned router: configuration should allow declaring access

[TinaHeiligers]

This issue is to handle the work needed for step 2 in
[http] Prevent access to internal-only APIs when running in serverless

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

Adapt the versioned router specification

Adapt RouteConfigOptions to:

• Add an access parameter at the route's level
• the parameter is required and can be set to either public or internal
• the parameter cannot be changed between versions

[elasticmachine]

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

[TinaHeiligers]

depends on #152269