add new route to delete items

[npracht]

Q	A
Bug fix? (use the a.b branch)	[ ]
New feature/enhancement? (use the a.x branch)	✅
Deprecations?	[ ]
BC breaks? (use the c.x branch)	[ ]
Automated tests included?	[ ]
Related user documentation PR URL	mautic/mautic-documentation#...
Related developer documentation PR URL	mautic/developer-documentation#...
Issue(s) addressed	Fixes #...

Description:

It was not possible to delete items by API. We introduce a new endpoint.

Steps to test this PR:

1. Open this PR for testing locally (see docs on testing PRs here)
2. use the API route delete Custom Items, GET /api/custom/item/delete/{itemId}

HTTP basic request example (using phpstorm):

POST http://localhost:8084/index_dev.php/oauth/v2/token
Accept: application/json,
Content-Type: application/json

{
    "client_id": "{{client_id}}",
    "client_secret": "{{client_secret}}",
    "grant_type": "client_credentials"
}

> {%
    client.global.set("auth_token", response.body.access_token);
%}

### Edit
PUT http://localhost:8084/index_dev.php/api/contacts/34690/edit
Content-Type: application/json
Authorization: Bearer {{auth_token}}

{
    "customObjects": {
        "data": [
            {
                "id": 2,
                "alias": "programmes",
                "data": [
                    {
                        "id": 18,
                        "name": "Hello From API2",
                        "attributes": {
                            "email": "[email protected]"
                        }
                    }
                ]
            }
        ]
    }
}

### Edit
GET http://localhost:8084/index_dev.php/api/custom/item/delete/19
Content-Type: application/json
Authorization: Bearer {{auth_token}}

@escopecz i think we need to add some of this info in the wiki as well, but it seems we cannot commit wiki code.

[Moongazer]

Would be super great if we'd be able to delete items via API, please merge and release.

[escopecz]

Please add functional tests to get this merged.