InRelease file: support Signed-By field

https://wiki.debian.org/DebianRepository/Format#Signed-By says:
> **Signed-By**
> An optional field containing a comma separated list of
> OpenPGP key fingerprints to be used for validating
> the next Release file. The fingerprints must consist
> only of hex digits and may not contain spaces.
> The fingerprint specifies either the key the Release file
> must be signed with or the key the signature key must be
> a subkey of. The later match can be disabled by appending
> an exclamation mark to the fingerprint.
>
> If the field is present, a client should only accept future updates
> to the repository that are signed with keys listed in the field.
> The field should be ignored if the Valid-Until field is not present
> or if it is expired.

For both the CLI tools and JSON, the field is taken as a string verbatim.

When specified, we must also provide `Valid-Until` field,
and i'm not sure there is an 'infinity' value for it,
so 100 years will have to do?

Fixes https://github.com/aptly-dev/aptly/issues/1497

cmd/publish_snapshot.go

@@ -150,6 +150,10 @@ func aptlyPublishSnapshotOrRepo(cmd *commander.Command, args []string) error {
 		published.AcquireByHash = context.Flags().Lookup("acquire-by-hash").Value.Get().(bool)
 	}
 
+	if context.Flags().IsSet("signed-by") {
+		published.SignedBy = context.Flags().Lookup("signed-by").Value.String()
+	}
+
 	if context.Flags().IsSet("multi-dist") {
 		published.MultiDist = context.Flags().Lookup("multi-dist").Value.Get().(bool)
 	}
@@ -247,6 +251,7 @@ Example:
 	cmd.Flag.String("codename", "", "codename to publish (defaults to distribution)")
 	cmd.Flag.Bool("force-overwrite", false, "overwrite files in package pool in case of mismatch")
 	cmd.Flag.Bool("acquire-by-hash", false, "provide index files by hash")
+	cmd.Flag.String("signed-by", "", "an optional field containing a comma separated list of OpenPGP key fingerprints to be used for validating the next Release file")
 	cmd.Flag.Bool("multi-dist", false, "enable multiple packages with the same filename in different distributions")
 
 	return cmd