`events_mgmt`: allow the use of custom Feign request interceptor

Refactor the `ims`, `core` and `events_mgmt` module to allow the user of this sdk to use his own Feign request interceptor

Start by reading the `ims`, `core`, and `events_mgmt` modules to locate how Feign request interceptors are configured. Trace the SDK client entry points and determine how a user-provided interceptor should be accepted across all three modules; done means custom interceptors can be supplied consistently without breaking the existing configuration.

Written by the indexing model from the issue text.