Add imageSourceProps to MessageImage

[amhinson]

There is currently no way to add additional headers to the image shown in MessageImage. This can be an issue if you need to add credentials to the header when making a request, for instance.

Usage:

<MessageImage
    {...props}
    imageSourceProps={{
        headers: {
            Authorization: `Bearer ${authToken}`
        }
    }}
/>

[gajjardarshithasmukhbhai]

Hi @amhinson, it's looking good to me. but we will wait for @Johan-dutoit.