Release file: support Version field

https://wiki.debian.org/DebianRepository/Format#Version

The Version field, if specified, shall be the version of the release.
On the other hand, if not set or set to an empty value, the Version
field will not be included in the Release file.

Signed-off-by: Zhang Xiao <xiao.zhang@windriver.com>

deb/publish.go

@@ -55,6 +55,7 @@ type PublishedRepo struct {
 	Label                string
 	Suite                string
 	Codename             string
+	Version              string
 	// Architectures is a list of all architectures published
 	Architectures []string
 	// SourceKind is "local"/"repo"
@@ -526,6 +527,7 @@ func (p *PublishedRepo) MarshalJSON() ([]byte, error) {
 		"Origin":               p.Origin,
 		"Suite":                p.Suite,
 		"Codename":             p.Codename,
+		"Version":              p.Version,
 		"NotAutomatic":         p.NotAutomatic,
 		"ButAutomaticUpgrades": p.ButAutomaticUpgrades,
 		"Prefix":               p.Prefix,
@@ -1080,6 +1082,9 @@ func (p *PublishedRepo) Publish(packagePool aptly.PackagePool, publishedStorageP
 				if p.SignedBy != "" {
 					release["Signed-By"] = p.SignedBy
 				}
+				if p.Version != "" {
+					release["Version"] = p.Version
+				}
 
 				var bufWriter *bufio.Writer
 				bufWriter, err = indexes.ReleaseIndex(component, arch, udeb).BufWriter()
@@ -1158,6 +1163,9 @@ func (p *PublishedRepo) Publish(packagePool aptly.PackagePool, publishedStorageP
 		// Let's use a century as a "forever" value.
 		release["Valid-Until"] = publishDate.AddDate(100, 0, 0).Format(datetime_format)
 	}
+	if p.Version != "" {
+        release["Version"] = p.Version
+    }
 	release["Description"] = " Generated by aptly\n"
 	release["MD5Sum"] = ""
 	release["SHA1"] = ""