feat: allow to pass custom icon for message input send button

[MartinCupela]

Motivation Add a new prop that would allow integrators to pass a component rendering the contents of the MessageInput's SendButton. At the moment it is necessary to replace the whole SendButton component. Keeping the SendButton component would allow to keep styling targeting the button and only its content would be changed. That would made the integration a bit more seamless.

Acceptance Criteria Possibility to pass a custom component rendering the SendButton contents through component context or MessageInput props.