search: introduce query mapper and lowercase transformer

Created by: rvantonder

Some visitor frameworks include a convenience mapper interface to transform trees. This is the initial mapper interface for and/or queries. The first use is a transformer that converts fields to lowercase, which is a pass that we currently do when validating current queries.

This is prep work for more and/or query validation in follow up PR #9757

Created by: codecov[bot]

Codecov Report

Merging #9752 into master will increase coverage by 0.01%. The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master    #9752      +/-   ##
==========================================
+ Coverage   41.76%   41.78%   +0.01%     
==========================================
  Files        1337     1339       +2     
  Lines       73420    73442      +22     
  Branches     6625     6625              
==========================================
+ Hits        30665    30685      +20     
- Misses      39912    39914       +2     
  Partials     2843     2843              

Flag	Coverage Δ
#unit	41.78% <90.90%> (+0.01%)

Impacted Files	Coverage Δ
internal/search/query/mapper.go	88.88% <88.88%> (ø)
internal/search/query/transformer.go	100.00% <100.00%> (ø)