Implement service endpoint for fetching the list of image styles

[aro]

We need an admin endpoint which will be used to fetch the list of image styles (https://github.com/enonic/xp/issues/6617, https://github.com/enonic/xp/issues/6618).

• The list should contain a combined list of styles from predefined styles.xml and custom-defined styles.xml from all apps of the site
• Only styles for <image> elements
• Fields needed in response: -- name (string) -- display-name (string) -- type (string) -- requires (array of objects with two fields: name and displayName (optional)) -- params (object)
• Style name must be unique in the list
• CSS path must include app name, and be unique
• Order of priority is from the first to last app, predefined styles in the end. If there's a style called "my-style" in an app and in predefined styles.xml, it's the app's style that should be fetched. This way users can override predefined styles by defining a style of the same name.

[aro]

Expected format:

{
    "css": ["assets/styles/styles.css"],
    "styles": [{
        "name": "editor-style-raw",
        "type": "style",
        "displayName": "Unprocessed raw image"
    }, {
        "name": "editor-align-justify",
        "type": "alignment",
        "displayName": "Justify",
        "requires": {
            "name": "editor-width-auto",
            "displayName": "Auto (100%)"
        }
    }, {
        "name": "editor-align-left",
        "type": "alignment",
        "displayName": "Align Left",
        "requires": {
            "name": "editor-width-auto",
            "displayName": "Auto (40%)"
        }
    }, {
        "name": "editor-align-center",
        "type": "alignment",
        "displayName": "Center",
        "requires": {
            "name": "editor-width-auto",
            "displayName": "Auto (60%)"
        }
    }, {
        "name": "editor-align-right",
        "type": "alignment",
        "displayName": "Align Right",
        "requires": {
            "name": "editor-width-auto",
            "displayName": "Auto (40%)"
        }
    }, {
        "name": "editor-width-auto",
        "type": "width",
        "displayName": "Auto"
    }, {
        "name": "editor-width-actual",
        "type": "width",
        "displayName": "Actual"
    }, {
        "name": "editor-width-custom",
        "type": "width",
        "displayName": "Custom"
    }, {
        "name": "editor-style-cinema",
        "type": "style",
        "displayName": "Cinema",
        "params": {
            "scale": "21:9"
        }
    }]
}

[aro]

Added a new url parameter in the config passed to main.html: stylesUrl To get the styles for a content, make a POST request to that URL passing one parameter: contentId