Add one config option (provider)

* "Issuer" config option for setting the CA issuer, these is used to
determine the valid TLS client certificates that allowed to access the
web-interface of the provider.

cmd/csaf_provider/config.go

@@ -44,6 +44,7 @@ type config struct {
 	DynamicProviderMetaData bool            `toml:"dynamic_provider_metadata"`
 	Publisher               *csaf.Publisher `toml:"publisher"`
 	UploadLimit             *int64          `toml:"upload_limit"`
+	Issuer                  string          `toml:"issuer"`
 }
 
 type tlp string