[ML] indicate ML node type in _cat/nodes

[elasticmachine]

Original comment by @inqueue:

I received a request to have ML nodes be indicated as such in the output of _cat/nodes; i.e., when node.ml: true . I am not sure I agree and maybe it is the use of "node" in ML documentation that is making people think it should be there.

I will continue recommending the use of GET _cat/nodeattrs to retrieve this info for now unless there is a better suggestion.

[elasticmachine]

Original comment by @droberts195:

To modify the output of _cat/nodes would require a change to core ES to allow plugins to register new node types.

I suggest collecting a list of customers who ask for this, then open a request in core ES if that list becomes overwhelming.

[elasticmachine]

Original comment by @clintongormley:

Given that we specify node.ml in the same way that we specify node.ingest, it makes sense to expose ML as a node type in _cat/nodes, so we need some sort of extension point which allows plugins to register node types for this API

[droberts195]

Following the work of #43175 this is now relatively straightforward.

The only remaining question is what the single letter abbreviation for "machine learning node" should be. It cannot be "m", because that is already taken by "master" nodes.

[pheyos]

The only remaining question is what the single letter abbreviation for "machine learning node" should be. It cannot be "m", because that is already taken by "master" nodes.

I would suggest to use a for analytics or advanced analytics (it would also match anomaly detection, but since we're adding more analytics this seems outdated to me).