[ML] Delete forecast API

[sophiec20]

Forecasts are currently deleted when:

• the job is deleted
• or, as part of midnight maintenance if the expires_in value is exceeded (14d default)

The possibility to delete a single forecast would enhance usability of the forecasting feature.

This could be useful in some scenarios, for example if a typo in the forecasting time range happened like 100d instaed of 10d.

Follow on UI changes - elastic/kibana#18511

[elasticmachine]

Pinging @elastic/ml-core

[droberts195]

Closed by #31134.