Generalise S3 buckets

[tonytw1]

What does this change?

Expands the concept of an S3 bucket from just the bucket name to a case class fully describing the bucket.
Allows GCP and locally hosted buckets to be used in addition to AWS.

Introduces an S3Bucket class to fully describe a bucket.
The definition of a bucket in configuration moves from bucket name to a triple of name, end point and URL style.

ie.

publishing.image.bucket.name="test-grid-crops"
publishing.image.bucket.endpoint="s3.amazonaws.com"
publishing.image.bucket.pathStyleURLs=false

Decisions about URLs of buckets and keys move to the S3Bucket class which has now enough knowledge to determine this.

How should a reviewer test this change?

How can success be measured?

Who should look at this?

Tested? Documented?

• locally by committer
• locally by Guardian reviewer
• on the Guardian's TEST environment
• relevant documentation added or amended (if needed)