Add method to get metadata from GCS blob in GCSHook

[jalengg]

• Background: GCS blobs can be associated with metadata to make tracking and validating data easier. The GCSHook doesn't yet allow for the metadata attribute to be exposed, although it supports uploading of blobs and associating metadata with them.
• Add get_metadata method to GCSHook
• The get_metadata function would retrieve metadata from blobs which already reside in a GCS bucket.

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[molcay]

LGTM

Just a small thing: it can be good to squash the commits

[potiuk]

LGTM I just rebased the branch. Lets confirm CI is green

Looks good.

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Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.