Switch to failover_buckets from failover_regions

[louiscb]

• Update ex_aws_s3 to include bug fix discovered recently
• Allows users to set a tuple of bucket and region version (since buckets are globally unique)
• Example

          [
            fetcher_mod: @fetcher,
            fetcher_opts: [
              bucket: "mo1212net-deals",
              region: "us-east-1",
              key: "hg.html",
              failover_buckets: [
                                [bucket: "adaptmx-airflow", region: "us-east-2"]
              ]
            ],

[sneako]

let's define a failover_bucket as a new type

[sneako]

maybe even types for bucket and region since we use them more than once

[sneako]

can you define a a schema for the failover buckets, so that nimble options validates it has the correct key/values?

[sneako]

Suggested change

	failover_buckets: failover_buckets
	failover_buckets: [_|_] = failover_buckets

just to make sure the list isn't empty. We can get rid of the guard with this change too