Licensing plugin improvements

[joshdover]

Meta issue to track a number of improvements to be made to the licensing plugin in X-Pack:

• Client-side implementation of feature usage API Client-side implementation of licensed feature usage API #72572
• Improve the feature usage API Improve Licensing plugin API #64619
• Add license-aware router interface Build license-checking into the New Platform #56926
• Make license available during client-side setup [Licensing] Enabling and disabling functionality based on current license in public #55985

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[pgayvallet]

Note that #72572 is required for #72019 that is due for 7.10

[pgayvallet]

Only one task remains uncompleted, so I'll go ahead and close this, #55985 can be tracked directly from its issue