Allow for attaching custom attachments in message input

[benlammers]

Motivation

Would like to have the ability to add custom attachments while drafting a message similar to the experience of attaching a file or image. The docs outline the ability to send a message with a custom attachment but not to store an attachment on the input state.

Proposed solution

Add a function to the MessageInputContext for setAttachments and expose attachments, so that custom attachments can be added and removed when drafting a message before sending it.

Acceptance Criteria

Ability to attach custom attachments to a message when drafting it, to be able to build an experience similar to attaching files or images but with custom attachments.