Nested query filters

[TylerPachal]

According to the spec, you can filter on included resources by doing something like:

/people?filter[car][make]=honda

When I try a request like this, everything works besides for the construction of the pagination urls.

This PR fixes the Utils.List.to_list_of_query_string_components/1 function so that its output is valid for the URI.encode_query/1 function.

---

Before this PR:

iex(1)> %{"filter" => %{"car" => %{"make" => "honda", "model" => "civic"}}} |> JSONAPI.Utils.List.to_list_of_query_string_components() |> URI.encode_query()
** (Protocol.UndefinedError) protocol String.Chars not implemented for %{"make" => "honda", "model" => "civic"} of type Map. This protocol is implemented for the following type(s): Atom, BitString, Cldr.Calendar.Duration, Cldr.Currency, Cldr.LanguageTag, Cldr.LanguageTag.T, Cldr.LanguageTag.U, Date, DateTime, Decimal, Float, Hex.Solver.Assignment, Hex.Solver.Constraints.Empty, Hex.Solver.Constraints.Range, Hex.Solver.Constraints.Union, Hex.Solver.Incompatibility, Hex.Solver.PackageRange, Hex.Solver.Term, Integer, List, NaiveDateTime, OpenApiSpex.Cast.Error, Phoenix.LiveComponent.CID, Postgrex.Copy, Postgrex.Query, Time, URI, Version, Version.Requirement
    (elixir 1.14.3) lib/string/chars.ex:3: String.Chars.impl_for!/1
    (elixir 1.14.3) lib/string/chars.ex:22: String.Chars.to_string/1
    (elixir 1.14.3) lib/uri.ex:153: URI.encode_kv_pair/2
    (elixir 1.14.3) lib/enum.ex:1755: anonymous fn/2 in Enum.map_join/3
    (elixir 1.14.3) lib/enum.ex:4289: Enum.map_intersperse_list/3
    (elixir 1.14.3) lib/enum.ex:1755: Enum.map_join/3
    iex:2: (file)

After this PR:

iex(1)> %{"filter" => %{"car" => %{"make" => "honda", "model" => "civic"}}} |> JSONAPI.Utils.List.to_list_of_query_string_components() |> URI.encode_query()
"filter%5Bcar%5D%5Bmake%5D=honda&filter%5Bcar%5D%5Bmodel%5D=civic"

[TylerPachal]

Annotation: This fix was just to make a recursive call, so that nested maps can be handled.