Minor UI improvements to per-element-metadata sample

[TaylorKrusen]

Description

Made some minor changes to add clarity to UX / UI

• Toast messages on add / remove metadata
• Input cleanup after removal
• Regrouped buttons and added secondary styles
• "Add Metadata" errors render in UI so users can see API limits

How Has This Been Tested?

Running the sample locally while making these changes

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[undavide]

Since you're here, would it be possible to inject a minimal amount of Spectrum/custom CSS to avoid serif fonts in the radio-buttons, toast, and input labels? The output textarea seems to have the wrong width/margin-right.

Functionally, it's awesome, thanks for adding that!!

[TaylorKrusen]

Updated screenshot with styling fixes: