Allow custom attach icon

[nicolethoen]

The default attach icon in the message bar can stay the paperclip attachment, but since we allow people to customize the menu, we should allow a custom icon to be in that attachment menu toggle to something to match their menu if needed.

This will support the following flow fidelity proposed Genie pattern: https://www.figma.com/design/lJ5s1z11E4Q2Bp6kD3TE0W/Red-Hat-Genie---MVP?node-id=676-17769&m=dev