* [ML][Data Frame] adds new pipeline field to dest config (#43124)
* [ML][Data Frame] adds new pipeline field to dest config
* Adding pipeline support to _preview
* removing unused import
* moving towards extracting _source from pipeline simulation
* fixing permission requirement, adding _index entry to doc
* adjusting for java 8 compatibility
* adjusting bwc serialization version to 7.3.0
Configurations are stored in the .data-frame-internal-1
index, but users should not add configurations directly to
the index as additional information to enable access control
is added. This adds a warning against allowing access to the
internal index.