Add functions for simple linear regression

[kkrik-es]

Calculate derivatives of gauges using simple linear regression, similar to deriv and predict_linear.

This likely requires multiple passes over the values in every bucket, first to calculate the mean values and then to apply the formula for simple linear regression.

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)

[pabloem]

The current state here:

• Deriv is implemented and merged 👍
• Predict_linear is a bit odd, so we are delaying merging it, however a PR for it is available here - Implementation of predict_linear for ESQL #137547

[kkrik-es]

Marking as completed for now. We can get back to predict_linear for promql compliance, at a later point.