[ML] Extend ML Forecast beyond 8 weeks in the REST API

[przemekwitek]

Borrowed from @boutcher:

"
Describe the feature:

Would like to be able to use the ML Forecast feature to forecast beyond 8 weeks. We understand that confidence in the predictions will be impacted as you move further into the future, but we have some data sets that have many years of historical data points that we believe could yield acceptable results without bumper guards at 8w.

Describe a specific use case for the feature:

We have ~10 years of daily event volume, and would like to do a forecast out 1-2 years to see what the model predicts. We understand that the accuracy isn't golden as you move further into the future.
"

[elasticmachine]

Pinging @elastic/ml-core

[przemekwitek]

Changes are required in 3 different places, the meta issue can be found here: #34054