fix(backend): able to preset filters on hidden columns & all queried

[ghiscoding]

• the backend services were using SlickGrid getColumns to get the column definitions but that returns only the visible columns, however we should be able to preset filters on hidden columns and also expect all columns to be queried (it's not because the column is hidden that we shouldn't be able to filter it and query it)

[codecov]

Codecov Report

Merging #768 (e10f723) into master (10e756c) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #768   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          160       160           
  Lines        11144     11145    +1     
  Branches      3980      3982    +2     
=========================================
+ Hits         11144     11145    +1     

Impacted Files	Coverage Δ
...lickgrid/components/angular-slickgrid.component.ts	100.00% <100.00%> (ø)
...ules/angular-slickgrid/services/graphql.service.ts	100.00% <100.00%> (ø)
...s/angular-slickgrid/services/grid-odata.service.ts	100.00% <100.00%> (ø)

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