Version all internal HTTP APIs

[kobelb]

Feature Description

To enable zero downtime upgrades, all internal HTTP APIs will need to be versioned. Any breaking changes to the HTTP APIs will need to result in a new major version of the HTTP APIs being released.

Business Value

Facilitates zero downtime rolling upgrades and rollbacks, allowing us to roll-out new features to our users more quickly while they continue to use the system without disruption.

Definition of Done

• All internal HTTP APIs are versioned
• HTTP API integration tests updated
• Unit tests

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)