grouping query builder and validator

[kkraune]

Creating a grouing query string (e.g. all( group(customer) each(output(sum(price))) ) ) can be difficult with multi-level grouping, having a UI to construct the string a la the querybuilder might help.

Can be useful as a validator, too - paste a string into a field, and auto-generate an pretty-printed view

hence both ways from structured to string and vice versa

see #16870 for implementation suggestion

[sjain166]

I like to work on this issue :)

[kkraune]

Sure, please refer to https://github.com/vespa-engine/vespa/blob/master/CONTRIBUTING.md for how to get started