Support CSEK for GCS Storage Backend

[salrashid123]

Support setting a CSEK key for GCS as a Storage Backend.
Vault will use provided Customer Supplied Encryption Key to encode the files it uses on Google Cloud Storage.

I'm unsure how to provide the unseal CSEK key to Vault to bootstrap the connection but as a small POC, here is a copy of the GCS backend where line 50 has the csek hardcoded

https://gist.github.com/salrashid123/e29c6af62e5780622d477c1d46237940
 
from there, the crud operations simply specifieds that key, for example

r, err := b.client.Bucket(b.bucket).Object(key).Key([]byte(csekKey)).NewReader(ctx)

in the end, you're left with CSEK sealed backend: