[7.x][ML] Machine learning data frame analytics (#43544)

[dimitris-athanasiou]

This merges the initial work that adds a framework for performing
machine learning analytics on data frames. The feature is currently experimental
and requires a platinum license. Note that the original commits can be
found in the feature-ml-data-frame-analytics branch.

A new set of APIs is added which allows the creation of data frame analytics
jobs. Configuration allows specifying different types of analysis to be performed
on a data frame. At first there is support for outlier detection.

The APIs are:

• PUT _ml/data_frame/analysis/{id}
• GET _ml/data_frame/analysis/{id}
• GET _ml/data_frame/analysis/{id}/_stats
• POST _ml/data_frame/analysis/{id}/_start
• POST _ml/data_frame/analysis/{id}/_stop
• DELETE _ml/data_frame/analysis/{id}

When a data frame analytics job is started a persistent task is created and started.
The main steps of the task are:

1. reindex the source index into the dest index
2. analyze the data through the data_frame_analyzer c++ process
3. merge the results of the process back into the destination index

In addition, an evaluation API is added which packages commonly used metrics
that provide evaluation of various analysis:

• POST _ml/data_frame/_evaluate

[elasticmachine]

Pinging @elastic/ml-core