Plugin should support Signing Profile ARN and Signing Profile Version ARN

At the moment of writing AWS CLI/Management and other SDK's support the following ARN formats:
- Signing Profile ARN: arn:aws:signer:::/**signing-profiles**/"
- Signing Profile Version ARN: arn:aws:signer:::/**signing-profiles**//"

However, AWS Signer Notation Plugin requires different ARN format:
- Signing Profile ARN: arn:aws:signer:::/**signing-profile**/"
- Signing Profile Version ARN: arn:aws:signer:::/**signing-profile**//"

It would be the best if the plugin supported the same ARN format as AWS CLI/Management/SDK.

Start by locating the Go plugin's ARN parsing or validation entry point, since the issue does not name a file or test. Compare its accepted signing-profile forms with the AWS CLI and SDK formats listed here; done means both signing profile ARN and signing profile version ARN formats are supported and covered by tests.

Written by the indexing model from the issue text.