Add Google Cloud Storage Backend

[kakkoyun]

Create a new backend implementation to support GCP Object storage

[a-hilaly]

Hello, can work on this one ?

[kakkoyun]

Hello @a-hilaly, of course, go ahead! You can bother me if you need anything. I'd be happy to review your work.

We already have interfaces and conventions in place to add a new back-end. You can have a look at https://github.com/meltwater/drone-cache/blob/master/cache/backend/s3.go to kickstart things.

[a-hilaly]

Nice 👍 ! Thanks for the tip @kakkoyun i'm going to open a PR soon

[spier]

I am doing some documentation cleanup, so adding this link here.

This issue is about integrating with https://cloud.google.com/storage/

[a-hilaly]

@kakkoyun while implementing this backend, i realised that Cloud Storage Go client need a context.Context for read/write operations. aws-go-sdk has also methods that uses context like PutObjectWithContext.

I had toughs about changing the Backend interface to allow context propagation with something like:

type Backend interface {
	Get(context.Context, string) (io.ReadCloser, error)
	Put(context.Context, string, io.ReadSeeker) error
}

Anyways this will certainly force changes all over the backends/plugin/main and need to be discussed/implemented in a separate PR.

I'm still not sure if context cancellation can be useful at this stage, but i think it can be a great feature, especially when dealing with huge objects.

[kakkoyun]

@a-hilaly That's a good idea. I'm also a fan of contexts nowadays. After we merge a couple of backend implementations, we can change the interface and refactor, accordingly if this emerges as a pattern.