Biggest change: we rename `Send` to `SendEvent` in `logical.EventSender`..
Initially we picked `Send` to match the underlying go-eventlogger
broker's `Send` method, and to avoid the stuttering of `events.SendEvent`.
However, I think it is more useful for the `logical.EventSender`
interface to use the method `SendEvent` so that, for example,
`framework.Backend` can implement it.
This is a relatively change now that should not affect anything
except the KV plugin, which is being fixed in another PR.
Another change: if the `secret_path` metadata is present, then
the plugin-aware `EventBus` will prepend it with the plugin mount.
This allows the `secret_path` to be the full path to any referenced
secret.
This change is also backwards compatible, since this field was not
present in the KV plugin. (It did use the slightly different `path`
field, which we can keep for now.)
The [WebSockets spec](https://www.rfc-editor.org/rfc/rfc6455) states
that text messages must be valid UTF-8 encoded strings, which protobuf
messages virtually never are. This now correctly sends the protobuf events
as binary messages.
We change the format to correspond to CloudEvents, as originally intended,
and remove a redundant timestamp and newline.
We also bump the eventlogger to fix a race condition that this code triggers.
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.
