Implementation of predict_linear for ESQL

[pabloem]

Implement a function to predict values for timeseries based on linear regression.
This PR depends on #137142 - which implements the deriv function.

Equivalent promql function: https://prometheus.io/docs/prometheus/latest/querying/functions/#predict_linear

This change:

• slightly modifies the underlying implementation of Deriv to take in a function to decide how to deal with linear regression prediction
• It relies on a window's last sample to predict the next upcoming value, so the result depends on the data distribution of the window.

[elasticsearchmachine]

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