feat: Added admin attribute functionalities

[jansentjeu]

This PR adds support in the Magento admin for assigning attributes to attribute landing pages based on data from Tweakwise. It introduces the option to use the Tweakwise backend API, allowing selection of attributes that are not part of the filter template.

TODO: This MR needs also #54 in new release

Testscenario 1:

1. Make sure the following setting is empty: Stores -> Configuration -> Catalog -> Tweakwise -> Attribute Landing Pages -> Backend API Access Token
2. Go to Content -> Attribute landing pages -> Manage pages
3. Add a new page and check if attribute functionality works the same as before
4. Edit an existing page and check if attribute functionality works the same as before

Testscenario 2:

1. Make sure your backend API access toke is set in: Stores -> Configuration -> Catalog -> Tweakwise -> Attribute Landing Pages -> Backend API Access Token
2. Go to Content -> Attribute landing pages -> Manage pages
3. Add a new page and check if not only attributes are showed which are available in your filter template
4. Edit an existing page and check if not only attributes are showed which are available in your filter template

[ahuininga-orisha]

When you add an custom attribute and value as an filter attribute, and you save the page. And open it again, the custom attribute and custom value is not displayed in the text field.

[jansentjeu]

When you add an custom attribute and value as an filter attribute, and you save the page. And open it again, the custom attribute and custom value is not displayed in the text field.

@ahuininga-orisha Fixed this issue. You have to pull newest changes from all modules.