Form - Allow multiple choice for dropdown

### Description

I would like to add multiple choice option for a dropdown menu in forms, as far as I know this feature is currently not available.
#### [Descussion on node-red forum](https://discourse.nodered.org/t/allow-multiple-choice-for-dropdown-in-forms/101419)

### Have you provided an initial effort estimate for this issue?

I am no FlowFuse team member

Start with the linked Node-RED forum discussion to understand the requested multiple-choice behavior, then locate the dashboard form dropdown implementation. Define the expected selection and submission behavior before changing it, and verify the result through the existing form UI checks or manual form interaction if no specific test is identified.

Written by the indexing model from the issue text.