feat: Update redshift_metadata_extractor to extract external schema

[jackgene]

This updated the redshift_metadata_extractor to extract external schema as well as internal ones.

amundsen-io/amundsen#1002

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[jackgene]

Hey guys,
I've made several attempts to sign off my last commit, but GitHub doesn't seem happy. Please let me know what I need to do.

Thanks!

[feng-tao]

@jackgene I think it requires all the commits to be sign, could you squash all the commits into single one and then sign on that commit? thanks

[jackgene]

@jackgene I think it requires all the commits to be sign, could you squash all the commits into single one and then sign on that commit? thanks

Thanks for getting back to me so quickly, Tao! I'll give it a try!