## Issue:
This PR adds an "EXISTS" condition to the event query grammar. It enables querying for the occurrence of an event without having to provide a condition for one of its attributes.
As an example, someone interested in all slashing events might currently catch them with a query such as slash.power > 0.
With this PR the event can be captured with slash.power EXISTS or just slash EXISTS to catch by event type.
## Examples:
`slash EXISTS`
## Commits:
* Add EXISTS condition to query grammar
* Gofmt files
* Move PEG instructions out of auto-generated file to prevent overwrite
* Update libs/pubsub/query/query.go
Co-Authored-By: Anton Kaliaev <anton.kalyaev@gmail.com>
* Update changelog and add test case
* Merge with other changes in PR #4070
* Add EXISTS to Conditions() func
* Apply gofmt
* Addressing PR comments
