[ML] Transforms UI only allows "histogram" on long fields - should also allow "terms"

[toby-sutor]

Part of #123459

Kibana version: 7.17 - 8.5.3

Describe the bug:
While creating Transforms via the Kibana UI, we cannot select terms aggregations for long fields. The only option available for long fields is histogram, which results in different output than terms.
Using the Elasticsearch API, we can create transforms on long fields in either terms or histogram. The expected behavior is that Kibana also offers both aggregation methods.

Steps to reproduce:

1. Deploy the Kibana weblogs sample data
2. Go to Stack Management -> Transforms and create a new transform
3. For Groupy by select terms(clientip) and another one histogram(machine.ram)
4. Notice how no terms(machine.ram) are available. Also, notice how the field cannot be changed via the Edit button

Expected behavior:
Kibana should offer other aggregations, too, e.g., terms.

Screenshots (if relevant):

Workaround:
The currently confirmed workaround is to use the Kibana UI with the histogram aggregation until the end of the transforms configuration page. There, it offers to Copy to clipboard, which copies the current API call. Paste this to the Kibana dev console and replace the histogramaggregation with aterms. aggregation. Note that the aggregation name must not contain a period (.). This allows for the setup of transforms with termsonlong` fields.

[elasticmachine]

Pinging @elastic/ml-ui (:ml)