[ML] Transforms: List should highlight when transform role is missing

[peteharverson]

Transforms require a transform node to run on. The Transform list should check whether there is a transform node, and if not, add a callout to the page warning them to configure at least one node with the transform role. Additionally the page could show the count of nodes with the transform role.

To verify whether there is a node that can run transforms, the output of GET _cat/nodes can be checked, so that at least 1 node has a t e.g.

v.x.y.z 2 99 2 0.68 0.66 1.43 dt  * elasticsearch03

This is of particular benefit now that Security includes a transform by default, so all new users are using transforms, and after a change to the cluster, such as a rolling upgrade or fine-tuning of settings, users may forget to set the transform role as they are unaware that they are using one.

Related to elastic/elasticsearch#69260.

[elasticmachine]

Pinging @elastic/ml-ui (:ml)