Get desktop application:
View/edit binary Protocol Buffers messages
Calls Signer.go->FetchUrl method.
URL to fetch from publisher's internal server. ie. PublisherServer.
URL to sign in sxg package.
ECC private key.
X509 certificate which may or may not contain CanSignHTTPExchange extension.
Extra request headers to pass to Packager. singer.go->ServeHTTP.
application/cert-chain+cbor format certificate response.
HTTP headers returned by the packager.