Add support for AWS STS endpoint in the Bucket API

[matheuscscp]

Fixes #1423

I was able to test this in my personal AWS account using both an interface endpoint and the public regional endpoints.

[matheuscscp]

Back to draft, adding tests for proxy

[stefanprodan]

LGTM

Thanks @matheuscscp 🏅

[darkowlzz]

Other than the last few suggestions about tests, the implementation looks good to me.
The STS mock tests particularly look great.

[darkowlzz]

I did some manual testing on EKS with IRSA.

• Without IRSA annotation, the bucket just failed due to no S3 permission.
• With IRSA annotation and no explicit STS config, just .spec.provider: aws, it worked.
• With explicit STS endpoint https://sts.amazonaws.com, it worked.
• Tried using a regional endpoint https://sts.us-east-2.amazonaws.com, it worked again.
• Tried a non-existing endpoint https://sts.us-east-5.amazonaws.com, it resulted in error Post \"https://sts.us-east-5.amazonaws.com\": dial tcp: lookup sts.us-east-5.amazonaws.com on 172.20.0.10:53: no such host.
• Tried a regional endpoint that's inactive in the account, resulted in error No OpenIDConnect provider found in your account for <OIDC provider address>.
• Tried http://sts.amazonaws.com and it failed with connection refused error.

Everything looks good and as expected.
Thanks for all the work on this and being patience with me 🙂

[matheuscscp]

I did some manual testing on EKS with IRSA.

• Without IRSA annotation, the bucket just failed due to no S3 permission.
• With IRSA annotation and no explicit STS config, just .spec.provider: aws, it worked.
• With explicit STS endpoint https://sts.amazonaws.com, it worked.
• Tried using a regional endpoint https://sts.us-east-2.amazonaws.com, it worked again.
• Tried a non-existing endpoint https://sts.us-east-5.amazonaws.com, it resulted in error Post \"https://sts.us-east-5.amazonaws.com\": dial tcp: lookup sts.us-east-5.amazonaws.com on 172.20.0.10:53: no such host.
• Tried a regional endpoint that's inactive in the account, resulted in error No OpenIDConnect provider found in your account for <OIDC provider address>.
• Tried http://sts.amazonaws.com and it failed with connection refused error.

Everything looks good and as expected. Thanks for all the work on this and being patience with me 🙂

Thanks for your excellent diligence, it keeps Flux being the best GitOps product out there!