Event API improvements

* Improvements to `Event.getField`
* [Allow default value `getField(ref, defaultValue)` for returning a default value if a field does not exist in the Event.](https://github.com/elastic/logstash/issues/8304)
* [Make `getField` throw exception if the requested field is missing.](https://github.com/elastic/logstash/issues/8305)
* [Provide typed `get` methods](https://github.com/elastic/logstash/issues/8303)
* Provide scalar -> list coercion on `getAsList`
* [Add Map-like `compute`, `computeIfAbsent`, and `computeIfPresent`.](https://github.com/elastic/logstash/issues/8306)
* Encourage `Event.getField(FieldReference)` (instead of `Event.getField(String)`)
* [Add FieldReference builder API (similar in idea to Java's Paths and Path api)](https://github.com/elastic/logstash/issues/8307)

These were the result of a discussion today that had an opening question of: "Can we improve the Event API by adding typed getters"