[Suggestion] A (public) function that takes the same args as `subset` and returns the matched indices

It would be great if DataFrames.jl had a function or functions that would function more or less the same way subset does, except that they'd would return a vector containing the indices of kept rows instead of a new frame. This vector would be suitable for subsequent row indexing. (Thankfully this function already more or less exists already.) For example, you'd have something like this:

julia> df = allcombinations(DataFrame, Symbol("col 1")=>1:5, Symbol("col 2")=>1:5); df[!, "col 3"] = missings(String, nrow(df)); df
25×3 DataFrame
 Row │ col 1  col 2  col 3   
     │ Int64  Int64  String? 
─────┼───────────────────────
   1 │     1      1  missing 
   2 │     2      1  missing 
   3 │     3      1  missing 
  ⋮  │   ⋮      ⋮       ⋮
  23 │     3      5  missing 
  24 │     4      5  missing 
  25 │     5      5  missing 
              19 rows omitted

julia> #= current way (BitVector) =# (df[!, "col 1"] .% 2 .== 0) .&& (df[!, "col 2"] .% 2 .== 1)
25-element BitVector:
 0
 1
 0
 1
 0
 ⋮
 0
 1
 0
 1
 0

julia> #= current way (indices) =# findall((df[!, "col 1"] .% 2 .== 0) .&& (df[!, "col 2"] .% 2 .== 1))
6-element Vector{Int64}:
  2
  4
 12
 14
 22
 24

julia> subset_conditions(df, selectors...; skipmissing::Bool=false, threads::Bool=true) =
           DataFrames._get_subset_conditions(df, Ref{Any}(selectors), skipmissing, threads);

julia> #= proposed way =# subset_conditions(df, "col 1" => c -> c .% 2 .== 0, "col 2" => c -> c .% 2 .== 1)
25-element BitVector:
 0
 1
 0
 1
 0
 ⋮
 0
 1
 0
 1
 0

julia> subset_indices(df, selectors...; skipmissing::Bool=false, threads::Bool=true) = 
           findall(DataFrames._get_subset_conditions(df, Ref{Any}(selectors), skipmissing, threads));

julia> #= proposed way =# subset_indices(df, "col 1" => c -> c .% 2 .== 0, "col 2" => c -> c .% 2 .== 1)
6-element Vector{Int64}:
  2
  4
 12
 14
 22
 24

Since these are suitable for indexing, you can do something like this:

julia> df[subset_indices(df, "col 1" => c -> c .% 2 .== 0, "col 2" => c -> c .% 2 .== 1), "col 3"] .= "even,odd"; df
25×3 DataFrame
 Row │ col 1  col 2  col 3    
     │ Int64  Int64  String?  
─────┼────────────────────────
   1 │     1      1  missing  
   2 │     2      1  even,odd
   3 │     3      1  missing  
  ⋮  │   ⋮      ⋮       ⋮
  23 │     3      5  missing  
  24 │     4      5  even,odd
  25 │     5      5  missing  
               19 rows omitted

For a simple example like this not much is gained, but for more complicated functions I think it begins to be worth it — especially if you have ByRow transformations that are tricky to express with broadcasting.

Start by reviewing the public subset API and the internal DataFrames._get_subset_conditions function mentioned in the issue. Determine how a public function should mirror subset's selectors and keyword arguments while returning row indices; done means the behavior and returned indices are covered by relevant tests.