[ML] Should be possible to update DFA memory estimate after adding exclude fields

[droberts195]

This is a follow-on from #60486. In the example job of that issue the original memory estimate was 644.9mb. To bring this down I added exclude fields. However, the estimate did not change when I did this. After adding quite a few exclude fields I manually reduced the model memory limit to 410mb, created the job, and started it. This revealed that the estimate with the excluded fields had reduced to 559.7mb.

However, having to create the job before being able to see the effect of the exclude fields will cause frustration. Maybe it's too much to re-estimate memory use on every edit to the exclude fields, but there should be a way to do it (e.g. 2 seconds after the last change to exclude fields or on clicking a button to explicitly ask).

[elasticmachine]

Pinging @elastic/ml-ui (:ml)