Add ValidationError formatter

[NickNeck]

This PR adds functionalities to customise the error messages for JsonXema.ValidationError.

JsonXema gets a new behaviour called JsonXema.ValidationError.Formatter and the def0ault formatter JsonXema.ValidationError.DefaultFormatter. This isolates the formatting to one module and the formatter module can be configured (e.g. config :json_xema, ValidationError, formatter: MyValidationErrorFormatter).

The JsonXema.ValidationError.DefaultFormatter excepts now options that are passed to the internal inspect(value, opts) calls. With this change all Inspect.Opts can be used with JsonXeam.ValidationError.format_error/2 (e.g. JsonXema.ValidationError.format_error(error, inspect_fun: my_inspect_fun)).

Examples:

iex(1)> schema = ~s|{"type": "integer"}| |> Jason.decode!() |> JsonXema.new()
%JsonXema{schema: %Xema.Schema{type: :integer}, refs: %{}}
iex(2)> error = JsonXema.validate(schema, %{one: 1})
{:error,
 %JsonXema.ValidationError{
   message: nil,
   reason: %{type: "integer", value: %{one: 1}}
 }}
iex(3)> JsonXema.ValidationError.format_error(error)
"Expected \"integer\", got %{one: 1}."
iex(4)> inspect_fun = fn value, _opts -> Jason.encode!(value) end
#Function<41.105768164/2 in :erl_eval.expr/6>
iex(5)> JsonXema.ValidationError.format_error(error, inspect_fun: inspect_fun)
"Expected \"integer\", got {\"one\":1}."
iex(6)> Application.put_env(:json_xema, ValidationError, inspect_fun: inspect_fun)
:ok
iex(7)> JsonXema.ValidationError.format_error(error)
"Expected \"integer\", got {\"one\":1}."

iex(1)> defmodule MyValidationErrorFormatter do
...(1)>   alias JsonXema.ValidationError
...(1)>
...(1)>   @behaviour JsonXema.ValidationError.Formatter
...(1)>
...(1)>   @impl true
...(1)>   def format(error, _opts) do
...(1)>     error
...(1)>     |> ValidationError.travers_errors([], &format_error/3)
...(1)>     |> Enum.reverse()
...(1)>     |> Enum.join("\n")
...(1)>   end
...(1)>
...(1)>   def format_error(error, path, acc) do
...(1)>     [inspect([error: error.reason, path: path], pretty: true) | acc]
...(1)>   end
...(1)> end
{:module, MyValidationErrorFormatter,
 <<70, 79, 82, 49, 0, 0, 8, 244, 66, 69, 65, 77, 65, 116, 85, 56, 0, 0, 1, 78,
   0, 0, 0, 31, 33, 69, 108, 105, 120, 105, 114, 46, 77, 121, 86, 97, 108, 105,
   100, 97, 116, 105, 111, 110, 69, 114, 114, ...>>, {:format_error, 3}}
iex(2)> Application.put_env(:json_xema, ValidationError,
...(2)>   formatter: MyValidationErrorFormatter)
:ok
iex(3)> schema = ~s|{"type": "integer"}| |> Jason.decode!() |> JsonXema.new()
%JsonXema{schema: %Xema.Schema{type: :integer}, refs: %{}}
iex(4)> schema |> JsonXema.validate("one") |> elem(1) |> Exception.message() |> IO.puts()
[error: %{type: "integer", value: "one"}, path: []]
:ok

[coveralls]

coverage: 98.864% (+0.05%) from 98.81%
when pulling df9310b on inspect-opts
into 3288577 on main.