[ML] DataFrame analytics config cannot be updated.

[przemekwitek]

Once created, the DataFrameAnalyticsConfig object is immutable.
It may be beneficial to give the user the ability to update it.
One possible use-case is updating model_memory_limit when the analyzed index had grown significantly since the last analysis.

[elasticmachine]

Pinging @elastic/ml-core

[droberts195]

Fixed by #58302