Time-series aggregation functions part 2

[jdcryans]

This is a continuation of #127446 to keep track of the remaining functions time-series aggregation functions that need to be implemented.

• irate (@pabloem) Implementing irate function for timeseries #134095
• increase (@pabloem ) Increase timeseries function #134934
• delta (@pabloem ) Implementation of Delta function for absolute change in gauges over time #135035
• idelta (@pabloem ) Implementing idelta function for esql #134510

[elasticsearchmachine]

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

[pabloem]

working on irate

[pabloem]

opened PR for rate - and due to synergies with other functions, also taking over implementation of idelta and delta.