Allow image to be selected from available term meta during taxonomy select

[cr0ybot]

In order to support adding an image block to the term template without the user having to write code, a better approach might be to allow the user to select a term meta key that is meant to be used as the image:

1. The user selects a taxonomy
2. The available registered meta keys for that taxonomy are presented
3. The user selects the meta key that corresponds to an image
4. The user optionally selects if the meta value corresponds to an attachment ID
5. The term template is filled with blocks and the image block is set to use the meta key selected

Number 4 might be confusing for some non-technical users, perhaps if an image block is using the term-meta binding for url there should be some code to auto-detect the value format and fetch an attachment URL if necessary?