Add support for expression chaining

[lbhm]

Please describe why this is necessary.

Since introducing basic statistics in #1235, we need a two-stage expression evaluation process in the commit path to first nest numRecords in a struct and then convert that struct to JSON. Currently, we cannot perform that in one step because the evaluation handler (or rather the ToJson expression) needs to know about the intermediate schema.

Describe the functionality you are proposing.

We could improve the situation by introducing expression chaining to the EvaluationHandler:

<input_schema> [--expression--> <output_schema>]+

This would allow us to link the two expression stages in the commit path. Fancy engines might decide to perform optimization rules to squash multiple chain links into one expression.

Additional context

No response