[Entity Client] Separate getting entities by id and get all definitions

[jennypavlova]

As discussed in this PR we want to separate the endpoints for getting the entity definitions.

cc: @miltonhultgren

AC:

• Based on the endpoint GET /internal/entities/definition/{id?} create 2 endpoints
  • GET /internal/entities/definition/{id} passing the definition id as a mandatory param
  • GET /internal/entities/definitions get all definitions (no definition id needed)

[elasticmachine]

Pinging @elastic/obs-ux-infra_services-team (Team:obs-ux-infra_services)

[elasticmachine]

Pinging @elastic/obs-entities (Team:obs-entities)