[Lens] Add expression data source

[flash1293]

To allow full flexibility in transforming data and fetching data from multiple sources at once, Lens should offer an "expression datasource". This allows to edit the underlying expression of the datasource part directly, while still keeping the visualization mapping as a simple to use drag and drop.

This is a generalization of the planned ESDSL data source (https://github.com/elastic/kibana/issues/59439) and should work in the same way: The user writes an expression, evaluates it and is able to drag and drop the columns of the resulting data table to chart dimensions.

Possible features:

• Start with expression created by default index pattern datasource - when switching over, the expression datasource is initialized with the current state of the index pattern datasource
• Save and load expressions independently from visualization - this allows reusing common logic in multiple visualizations (e.g. data analyst is preparing the expressions, end user is tying them to a chart for a nice dashboard)
• In addition to the previous point - Allow expressions to be parameterized by variables. This makes it possible to configure the independently saved expressions in different ways depending on the chart (e.g. switching between different aggregation functions and raw data, but applying the same post processing)

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[stratoula]

Thank you for contributing to this issue, however, we are closing this issue due to inactivity as part of a backlog grooming effort. If you believe this feature/bug should still be considered, please reopen with a comment.