[FeatureRequest] Blueprint: allow mediapicker to be bound to folder

**Description**

As an editor usting the grav-plugin-admin I would like to upload hero images and store them in a specified, common folder, so I can re-use the same image on another page.

Here, I want to choose exclusively from images in the common folder for quick selection.
This is currently possible only with `type: filepicker`, which gives a dropdown with very small previews.

Instead I would like to preview the images moderately large, as presented in `type: mediapicker`, so I can easily identify each image.

**Acceptance criteria**

* Blueprint fields of `type: mediapicker` exclusively retrieve images from `folder: 'image://foo'`, if `folder` is specified

**Further reading**

* Compare issue #3775
* Compare grav-plugin-admin issue [#1696 Bad usability adding and selecting media #1696](https://github.com/getgrav/grav-plugin-admin/issues/1696)

**Screenshot**

* Currently `type: mediapicker` always retrieves images from "All Pages"


Start by comparing this request with issue #3775 and the grav-plugin-admin issue #1696, then trace how Blueprint fields of type mediapicker currently retrieve images from All Pages. Done means a specified folder such as image://foo restricts the picker to images in that folder while retaining the larger mediapicker previews.

Written by the indexing model from the issue text.