github: Add dependency checker action

github: Add dependency checker action

Add an action, dependabot, that will periodically check GitHub Actions
dependencies.  It is currently configured to run weekly.  If dependabot
finds an update, it will open a pull request.

See here for more details:
	https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot

Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>
Reviewed-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>