Subscribing to events through a WebSocket now support boolean
expressions to filter only the events wanted based on the fields
* `event_type`
* `operation`
* `source_plugin_mount`
* `data_path`
* `namespace`
Example expressions:
These can be passed to `vault events subscribe`, e.g.,:
* `event_type == abc`
* `source_plugin_mount == secret/`
* `event_type != def and operation != write`
```sh
vault events subscribe -filter='source_plugin_mount == secret/' 'kv*'
```
The docs for the `vault events subscribe` command and API endpoint
will be coming shortly in a different PR, and will include a better
specification for these expressions, similar to (or linking to)
https://developer.hashicorp.com/boundary/docs/concepts/filtering
022469da45