Missing `SHA-256=` prefix in `Digest` header for request signing

[pengux]

The documentation states that the digest header should have the SHA-256= prefix. Although the code in Kong doesn't actually uses the value from the header in the request so the bug doesn't seem to trigger any error.

https://api-docs.form3.tech/tutorial-request-signing.html
https://github.com/form3tech-oss/go-form3/blob/master/pkg/form3/auth_request_signing.go#L98
https://github.com/form3tech/docker-kong/blob/develop/kong/form3-signer/handler.lua#L172