Add storage example for Google Cloud Storage

[malisetti]

Please add an example for GCS at https://perkeep.org/doc/storage-examples
It is very confusing to get the required configuration information from gcs.

[kalidor]

Did you look at https://perkeep.org/doc/server-config ? There is a line about GCS which seems pretty explicit:

googlecloudstorage: "clientId:clientSecret:refreshToken:bucketName[/optional/dir]"

[malisetti]

https://perkeep.org/doc/storage-examples describes how to get configuration information required from AWS and backblaze. I am only asking the same for GCS