Added dynamic bucket region session option

[pregnor]

Q	A
Bug fix?	no
New feature?	yes
API breaks?	no
Deprecations?	no
Related tickets	-
License	Apache 2.0

What's in this PR?

1. Added an AWS session initialization option for dynamically retrieving the region of the S3 bucket which is used in the helm command.
2. Added unit test cases covering all paths of this option.
3. Updated the calls to the session constructor to use this option.
4. Added end to end test cases for fetch operation using dynamic bucket region.
5. Fixed linter issues in the modified packages to ensure passing checks.

The HEAD {{bucket}} request allows querying the bucket's actual region without prior knowledge with the proper configuration.

Unfortunately this behavior is not explicitly documented - or at least I couldn't find it - but the AWS SDK Go team confirmed in an issue comment this is an intended and supported behavior of the endpoint.

If the option runs into any issue, it has no effect on the session/configuration as a fallback mechanism. In such a case the plugin's behavior remains the same as before, namely falling back to the HELM_S3_REGION environment variable's value or to the corresponding AWS environment variable's value (AWS_REGION or AWS_DEFAULT_REGION).

Why?

Without the correct region being set currently the plugin throws an error:

fetch from s3 uri=s3://pregnor-helm-s3-repo-test/stable/cadence/cadence-0.18.0.tgz: fetch object from s3: BucketRegionError: incorrect region, the bucket is not in 'us-west-2' region at endpoint ''
        status code: 301, request id: , host id: 
Error: plugin "bin/helms3" exited with error

We wanted to allow the plugin to work without manual region setting and also support multiple repositories residing in different regions without updating the regions in-between fetching charts from those repositories. This is especially relevant when the plugin is used in a Terraform environment.

Additional context

The buckets used in the unit tests were found randomly, they are not mine and I do not have access to them, but they fit the test cases perfectly.

Checklist

• Code meets the Developer Guide
• User guide and development docs updated (if needed)
• Related Helm chart(s) updated (if needed)