510 - SBOM attestation stdout (#785)

add syft attest command to produce an attestation as application/vnd.in-toto+json to standard out using on disk PKI

Signed-off-by: Christopher Phillips <christopher.phillips@anchore.com>

syft/encode_decode.go

@@ -17,6 +17,7 @@ func Encode(s sbom.SBOM, option format.Option) ([]byte, error) {
 	if f == nil {
 		return nil, fmt.Errorf("unsupported format: %+v", option)
 	}
+
 	buff := bytes.Buffer{}
 
 	if err := f.Encode(&buff, s); err != nil {