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.)
