Add support for AWS SSO

[antonosmond]

Annoyingly the AWS SDK for Go doesn't natively support credentials via SSO access tokens.
It does however provide the ability to create a custom credential provider - see here
This PR adds a custom credential provider capable of utilising the SSO token from the SSO token cache.
It's enabled via the env var AWS_SSO and relies on the AWS_PROFILE env var to get the SSO account ID and role name from the AWS config file in ~/.aws/config.

[b4nst]

Would love to see this implemented. Currently my workaround is to use aws2-wrap but it's really cumbersome

[urz9999]

In case like this one or other similar cases where AWS SSO result in incompatibilities with your library and you don't want to play with workarounds or complicated fixes, maybe you can give a try to our open-source project: https://github.com/Noovolari/leapp. It deals with AWS SSO authentication and accounts/roles retrieval then it creates short-lived temporary credentials in .aws/credentials to maximize compatibility with third party tools / sdks.

[antonosmond]

Closing this as AWS have finally added SSO support in the SDK:
https://github.com/aws/aws-sdk-go/releases/tag/v1.37.0
Once helm-s3 is updated to use the new SDK, it should work with SSO credentials