[Feature request] Allow multiple folder sources for Multiselect (Notes) field

**Is your feature request related to a problem? Please describe.** Currently, for Multiselect fields, you have three choices for the source of multiselect options. One is Notes which allows you to identify a folder as your source, such that all notes in that folder will be offered as options in the modal. But you can only identify one folder. **Describe the solution you'd like** I would like to be able to add multiple folders as Sources for a single multiselect field, so that when I open the modal form it offers me all notes in all the added folders as options. **Additional context** <!-- Add any other context or screenshots about the feature request here. -->