Support scripted pipeline calls

[peterpramb]

Describe the feature:

Currently only the pipeline processor can be used to execute other pipelines, but it currently does not support templating (see #39955).

To allow for dynamic pipeline chaining it would be helpful to introduce a new pipeline call in painless to explicitly trigger the execution of a specific pipeline. The expectation is that the call will return after execution as the pipeline processor does.

Would this be possible at all?

[elasticmachine]

Pinging @elastic/es-core-features

[dakrone]

This has been open for quite a while, and we haven't made much progress on this due to focus in other areas. For now I'm going to close this as something we aren't planning on implementing. We can re-open it later if needed.