feat: use google/go-cloud to fetch rules and credentials from object storage

[alekitto]

Related issue

#518

Proposed changes

As proposed in #518, I've used go-cloud library to fetch credentials and rules files from an object storage (S3, Azure Blob Storage or GCS).

Checklist

• I have read the contributing guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security. vulnerability, I
  confirm that I got green light (please contact
  [email protected]) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further comments

I'm unsure about how to test this feature: probably I can use mocks to unit test the fetcher, but probably an integration test could be more useful (with a minio service container to test real S3 api calls maybe?)

[aeneasr]

Yo, this is awesome! A couple of things that we could do to make this even better:

1. Add tests for azblob/gs/s3 - I think that gocloud has helpers to help you write tests
2. Document this feature here and here and here.

Thank you!

[aeneasr]

Great work, thank you! :)