Include `Signature-Agent` within `Signature-Input` both at signing and verification time

[thibmeu]

This allows Signature-Agent to be binded to the signature.

This has been included in draft-meunier-web-bot-auth-architecture in the following PR thibmeu/http-message-signatures-directory#15, to be cut in -01.

[leelynne]

I'll look to add support for Signature-Agent and the directory to https://github.com/remitly-oss/httpsig-go.

[thibmeu]

Thanks! Happy to review changes if you need.