This isn't perfect for sure, but it's solidifying and becoming a useful
base to work off.
This routes events sent from auth and secrets plugins to the main
`EventBus` in the Vault Core. Events sent from plugins are automatically
tagged with the namespace and plugin information associated with them.
For example, using:
```sh
vault server -dev -experiment events.beta1
```
Tested by checking that the events were enabled and disabled
when the `-experiment events.beta1` flag was present and absent.
Also added a small fix to pass the `hclog.Logger` in now so that
the logging hierarchy and levels are respected.
Creates a new `eventbus` package under `vault` with
an implementation of the `go-eventlogger` broker.
Also creates a stub of a common broker that will be accessible
in the core, and creates a simple event sending interface.
