[ML] Transforms Validation

[walterra]

https://github.com/elastic/elasticsearch/pull/68396 introduces header warnings for the _preview endpoint. This can be used to provide hints to users in the Transforms creation wizard. Possibly as an extra validation step or as part of the preview table.

Examples for warnings:

• Warning if all fields in the group_by are runtime fields (or script based, which is very similar). That means an ordinary terms agg together with a rt-based date histogram should trigger a warning.
• There's plans to check dest against potential template and mapping conflicts.
• If a runtime field is chosen as the timestamp field

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[peteharverson]

See also this issue, found during runtime fields investigatoin in #77462, when creating a date_histogram groupby transform preview by using a big dataset (like Nginx), a 500 internal server error was thrown. Preview through API is working fine. Note there are performance issues when the timestamp field of the index pattern is a runtime field. Possible solution may be to prevent the user selecting a runtime field as the timestamp field when creating a Kibana index pattern, or display a warning that performance may be impacted.