Dynamic variables for inputs

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From: https://gitlab.com/manticoresearch/clt/-/issues/5

While doing a scenario to test mysqldump with combined tasks no mcl, mcl only, mcl id with json, MVA, text fields, we found that it's a good idea to have the possibility to support dynamic variables in inputs. So we can use one similar test, for example, to run it several times just by changing the name of the dataset or something else that may be useful to reduce the amount of copy-paste of tests and improve maintenance.

We should think about the best way to use it, but probably the most straightforward idea to implement is just adding the possibility to export env variables into a shell run by Rust and use it in rec files. Still, we also need to modify comparing mechanism in that case.

We should research the best way first before we implement it.

Start by reviewing how rec files are parsed, how Rust launches the shell, and how comparison currently works. Research and document a variable syntax and its behavior for repeated scenarios, including dataset names and other inputs. Done means the approach is agreed and its required comparison changes and test coverage are defined.

Written by the indexing model from the issue text.