[Feature] Add optional parameter 'data' to execute_custom_step keyword

[osedlbauer]

I stumbled across this project and think it is really great. It works like a charm!

This PR adds an optional parameter 'data' to the 'Execute Custom Step' keyword.
The object is passed through to the Jinja2 template, allowing custom templates to handle more complex and dynamic dialog windows.

This is a small example of the use case this PR solves:

templates/complexform.html

{% extends "dialogs/layout.html" %}
{% block dialog_content %}

<h5>Enter Data</h5>

<form method="POST" action="/dialog/submit">

  {% for key, value in data.items() %}
  <div class="mb-3">
    <label class="form-label">{{ key }}</label>
    <input
      type="text"
      class="form-control"
      name="{{ key }}"
      value="{{ value }}">
  </div>
  {% endfor %}

  <button type="submit" class="btn btn-primary">Submit</button>
</form>
{% endblock %}

test.robot

*** Settings ***
Library    WebDialogs


*** Test Cases ***
Test Complex Form With Data
    VAR     &{testdata}
    ...    data1=value1
    ...    data2=${EMPTY}

    ${result}=    WebDialogs.Execute Custom Step
    ...    step=complexform
    ...    data=${testdata}

    Log    result:${result}    console=true