Add iTwin.js watermark component

[GerardasB]

Is your feature request related to a problem? Please describe.
Currently, the iTwin.js watermark can be obscured by widget panels and tools. The watermark should be treated as a bottom-aligned toolbar button (although use the borderless button with no frosted glass background effect https://itwinui.bentley.com/docs/button#borderlesS ).

Describe the solution you'd like
Instead of using the default watermark button provide a watermark button component that has the capability to display the "Powered by iTwin.js" modal dialog ( https://itwinui.bentley.com/docs/dialog#dismissible ).
The new dialog should have the capability to provide custom content.

Issues:

Align watermark to toolbars:

Current dialog:

New iTwinUI dialog: