feat: add Google Cloud Storage cache adapter

[matheusmatos]

This PR implements Google Cloud Storage as a new cache type.

To use it, you need to configure cache as the example below:

[cache]
# required
type="gcs"
bucket="your_bucket_name"   # Bucket is the name of the GCS bucket to operate on

# optional
basepath="tegola"           # Basepath is a path prefix added to all cache operations inside of the GCS bucket
max_zoom=8                  # MaxZoom determines the max zoom the cache to persist.

The credentials (service account and project_id) are handled by the GOOGLE_APPLICATION_CREDENTIALS environment variable.

[coveralls]

Pull Request Test Coverage Report for Build b5a95d2ec-PR-891

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 45.322%

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Totals
Change from base Build 01e9eb9a8:	0.0%
Covered Lines:	5745
Relevant Lines:	12676

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💛 - Coveralls

[ARolek]

Merged into master! Thank you for the contribution.

[kolisko]

@matheusmatos Thanks! Is possible to set up TTL for gcs cache in config file?