helm: Support providing S3 credentials via a secret

S3 is one option for cache export and credentials can be provided to buildkitd directly instead of relying on client's passing them in. They can also be provided that way but this let's us support a centralized cache as well.

See https://github.com/moby/buildkit#s3-cache-experimental See https://docs.aws.amazon.com/sdk-for-go/v1/developer-guide/configuring-sdk.html

Bug: T320746