Feature Request: add keyPattern option for addSourceBucket method

[naeemcloudguru]

This feature would allow the user to pass an optional key pattern to this method as a second argument so that in the case that the user is using a "global" bucket (bucket used by others but has a target object within the bucket), the user can specify the pattern to the target and give permissions to only update objects on trigger that are in that target. If there was no pattern passed, then the original '*' would be used for the keyPattern.

[dontirun]

I think scanPrefix would be a better name for this. Happy to take a PR!