Break up aws_signing_helper to make it possible to use this package without requiring CGO builds

[strideynet]

We're working on a tool to ease integration with AWS Roles Anywhere when using SPIFFE as a source of X509 credentials - https://github.com/spiffe/aws-spiffe-workload-helper. Our goal is to be able to distribute a static binary of this tool to ease portability.

Today, the aws_signing_package includes a dependency on https://github.com/miekg/pkcs11, as a result, any package which depends on aws_signing_package will therefore require CGO to build correctly. To achieve our goal, we have to partially vendor this package and patch it, which makes the process of upgrading to include the latest changes more challenging.

If the core functionality necessary to produce an AWS credential from a Signer was moved to a distinct package, we would be able to depend on this package without incurring the cost of requiring a CGO enabled build.

If this change sounds acceptable, I wouldn't mind producing a PR for this.