Add flux events command

This adds a command to Flux which behaves similarly as `kubectl events`,
including the Flux sources events when showing events for top level
objects, thus making debugging easy for Flux users.

For example, `flux events --for kustomization/<name>` includes the
events of its source (e.g. a `GitRepository`, `OCIRepository` or
`Bucket`).

In addition, `flux events --for helmrelease/my-app` includes events of
the `HelmChart` and `HelmRepository`. While `flux events --for
alerts/slack` includes the events of the `Provider`.

Signed-off-by: Somtochi Onyekwere <somtochionyekwere@gmail.com>