Top metrics/ first and last aggs in ES|QL

**Value Proposition**

ES|QL lacks First/Last aggregation functions that are commonly used in observability and security use-cases in order to emit the first/last observed metric value within a time window. This is most commonly based on a timestamp ordering but it can also be useful to get the first document based on another criteria, such as a numerical size/salary/length/distance etc.

**Expected Outcomes**

First and last functions to return the earliest or latest value of a field, respectively, when no sort criteria is provided (default to @timestamp). Optional sort criteria which could be a numeric field