Add support for adding an icon and a link to gr.Button()

[abidlabs]

It has come up several times, most recently while reviewing #4943, that we should add support for adding an icon to the gr.Button() component. For example, we could add an icon parameter which takes in a filepath or a URL to a svg/png file, which appears before the text itself.

Separately, we should also consider adding a link parameter to gr.Button() to open webpages upon button click. This could potentially be a separate LinkButton custom component