Add Google Cloud Storage support

[MarkEdmondson1234]

I tried to keep to the same format as the S3 integration.

I made a bucket on GCS called memoise-tests that I can give you access to as well for the tests to work as is (after you add your own authentication file) - just let me know your Google email to add it to. The tests all work for me locally, for them to work for you too it would need these authentication environment arguments set:

• GCS_AUTH_FILE - pointing at the service account JSON
• GCS_DEFAULT_BUCKET - pointing at the GCS bucket you have pre created, either on the web UI or via googleCloudStorageR::gcs_create_bucket()
• GAR_CLIENT_JSON to set your own client_id and secret via googleAuthR::gar_set_client() in the tests. It also assumes you have activated Cloud platform scope (on by default on GCPs) This is also downloaded from a Google Cloud project, another JSON file. See the help. Or you can set it directly via options(googleAuthR.client_id) and options(googleAuthR.client_secret) but I prefer the former.

The last one is only due to running non-interactivly in the tests, usually it will get set when you load library(googleCloudStorageR) but I didn't think that was a good idea in the tests.

I did deviate a bit by not creating a new bucket each time like that is done in the S3 function, I don't think thats necessary and its a lot smoother with a pre-made one. I also don't create a new bucket if its not there, I'd prefer more control when that is done, but you may disagree, I'll defer to whatever you think best.

Thanks for the package, its really useful, I use it for caching API calls in other packages.

[codecov-io]

Codecov Report

Merging #59 into master will decrease coverage by 42.13%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           master      #59       +/-   ##
===========================================
- Coverage     100%   57.86%   -42.14%     
===========================================
  Files           4        5        +1     
  Lines         151      197       +46     
===========================================
- Hits          151      114       -37     
- Misses          0       83       +83

Impacted Files	Coverage Δ
R/cache_gcs.R	0% <0%> (ø)
R/cache_s3.R	0% <0%> (-100%)	⬇️
R/memoise.R	100% <0%> (ø)	⬆️

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[jimhester]

Thanks! This looks fine, if you can just add a note to NEWS.md with the change and mentioning your GitHub username and this issue I can merge.

[MarkEdmondson1234]

Thanks @jimhester - added those bits now.

[jimhester]

Thanks!