[ML] add GET _cat/ml/datafeeds

[benwtrent]

This adds GET _cat/ml/datafeeds && _cat/ml/datafeeds/{datafeed_id}

Example calls

# GET _cat/ml/datafeeds?v
#! Deprecation: [interval] on [date_histogram] is deprecated, use [fixed_interval] or [calendar_interval] in the future.
id                             state bucket.count search.count
datafeed-low_request_rate    stopped 1457         3
datafeed-response_code_rates stopped 1460         18
datafeed-url_scanning        stopped 1460         18

# GET _cat/ml/datafeeds?h=*&v
#! Deprecation: [interval] on [date_histogram] is deprecated, use [fixed_interval] or [calendar_interval] in the future.
id                             state assignment_explanation bucket.count search.count search.time search.bucket_avg   search.exp_avg_hour node.id node.name node.ephemeral_id node.address
datafeed-low_request_rate    stopped                        1457         3            62ms        0.0425531914893617  20.160978098329824                                      
datafeed-response_code_rates stopped                        1460         18           127ms       0.08698630136986302 2.529951653336354                                       
datafeed-url_scanning        stopped                        1460         18           173ms       0.1184931506849315  2.8745993097731013                                      

# GET _cat/ml/datafeeds?help
id                     |                       | the datafeed_id                                           
state                  | s                     | the datafeed state                                        
assignment_explanation | ae                    | why the datafeed is or is not assigned to a node          
bucket.count           | bc,bucketCount        | bucket count                                              
search.count           | sc,searchCount        | number of searches ran by the datafeed                    
search.time            | st,searchTime         | the total search time                                     
search.bucket_avg      | sba,bucketTimeMin     | the average search time per bucket (millisecond)          
search.exp_avg_hour    | seah,searchExpAvgHour | the exponential average search time per hour (millisecond)
node.id                | ni,nodeId             | id of the assigned node                                   
node.name              | nn,nodeName           | name of the assigned node                                 
node.ephemeral_id      | ne,nodeEphemeralId    | ephemeral id of the assigned node                         
node.address           | na,nodeAddress        | network address of the assigned node                      

closes #51411

[elasticmachine]

Pinging @elastic/ml-core (:ml)

[davidkyle]

I thought this was GetDatafeeds not stats but stats makes a lot more sense 👍. I suppose naming the endpoint _cat/ml/datafeeds_stats isn't great. All good here.

[davidkyle]

Are there any byte units returned?

[benwtrent]

Don't think so, these options are a catch all as these parameters come by default with _cat APIs.

[davidkyle]

There is also a default size parameter https://www.elastic.co/guide/en/elasticsearch/reference/current/cat.html that is only used by cat thread pool

My preference would be to remove this but it's not a deal breaker

[davidkyle]

LGTM

[davidkyle]

There is also a default size parameter https://www.elastic.co/guide/en/elasticsearch/reference/current/cat.html that is only used by cat thread pool

My preference would be to remove this but it's not a deal breaker

[benwtrent]

@elasticmachine update branch

[lcawl]

Documentation added in #52738