feat: add --ignore-paths flag to flux create source (git|bucket)

A new --ignore-paths flag is added to following commands:

flux create source git --ignore-paths ...
flux create source bucket --ignore-paths ...

A StringSliceVar is used which supports specifying the flag multiple
times to populate a list or either a comma seperated string value

A unit test with a golden file is added to validate the flag

Signed-off-by: Tarun Gupta Akirala <takirala@users.noreply.github.com>