Consider adding a mechanism for running a script specified in userdata

[cbgbt]

What I'd like: The ability to specify a bootstrap script that gets run by an existing bottlerocket-provided bootstrap container.

Check out the discussion in #1989, particularly this comment

Any alternatives you've considered: Users can currently provide arbitrary bootstrap containers to accomplish the same outcomes, this would just make it easier, since it would eliminate the need to publish and maintain an additional container.