expose public licensing APIs from core

[TinaHeiligers]

Alternative implementation toward #187296.
Given that we cannot move the entire licensing plugin to core, we should consider if exposing frequently used licensing APIs from core, similarily to how authc and audit is exposed from core.security. We would need to delegate the implementation back to the licensing plugin.

cc @SiddharthMantri

[elasticmachine]

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

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)

[TinaHeiligers]

There isn't a benefit to exposing licensing APIs from core without moving the implementation. We cannot cross the code licensing border and will revisit if and when that changes.