Add Cargo package registry (#21888)

This PR implements a [Cargo registry](https://doc.rust-lang.org/cargo/)
to manage Rust packages. This package type was a little bit more
complicated because Cargo needs an additional Git repository to store
its package index.

Screenshots:

![grafik](https://user-images.githubusercontent.com/1666336/203102004-08d812ac-c066-4969-9bda-2fed818554eb.png)

![grafik](https://user-images.githubusercontent.com/1666336/203102141-d9970f14-dca6-4174-b17a-50ba1bd79087.png)

![grafik](https://user-images.githubusercontent.com/1666336/203102244-dc05743b-78b6-4d97-998e-ef76341a978f.png)

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

models/packages/descriptor.go

@@ -11,6 +11,7 @@ import (
 	repo_model "code.gitea.io/gitea/models/repo"
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/json"
+	"code.gitea.io/gitea/modules/packages/cargo"
 	"code.gitea.io/gitea/modules/packages/composer"
 	"code.gitea.io/gitea/modules/packages/conan"
 	"code.gitea.io/gitea/modules/packages/conda"
@@ -129,6 +130,8 @@ func GetPackageDescriptor(ctx context.Context, pv *PackageVersion) (*PackageDesc
 
 	var metadata interface{}
 	switch p.Type {
+	case TypeCargo:
+		metadata = &cargo.Metadata{}
 	case TypeComposer:
 		metadata = &composer.Metadata{}
 	case TypeConan: