JSON indexing: wildcard-like index over fulltext

This is Goal #2 of MDEV-40168

That is, an index containing every (path, value) pair of a json document. In other words, instead of a usual JSON MVI which is in the form of

create index idx1 on t1 ((cast(jscol->'$.tags' as DATATYPE array)));

a wildcard index is akin to

create index idx1 on t1 (jscol->'$.**');

MVI-on-fulltext naturally extends to this. Other JSON index backends could incur different engineering costs.

TODO: very brief mention of engineering costs of various backend designs as suggested by Claude

TODO: do we need JSON comparator as mentioned in MDEV-37288?