Implement Cargo HTTP index (#24452)

This implements the HTTP index
[RFC](https://rust-lang.github.io/rfcs/2789-sparse-index.html) for Cargo
registries.

Currently this is a preview feature and you need to use the nightly of
`cargo`:

`cargo +nightly -Z sparse-registry update`

See https://github.com/rust-lang/cargo/issues/9069 for more information.

---------

Co-authored-by: Giteabot <teabot@gitea.io>

routers/api/packages/cargo/cargo.go

@@ -4,6 +4,7 @@
 package cargo
 
 import (
+	"errors"
 	"fmt"
 	"net/http"
 	"strconv"
@@ -45,6 +46,35 @@ func apiError(ctx *context.Context, status int, obj interface{}) {
 	})
 }
 
+// https://rust-lang.github.io/rfcs/2789-sparse-index.html
+func RepositoryConfig(ctx *context.Context) {
+	ctx.JSON(http.StatusOK, cargo_service.BuildConfig(ctx.Package.Owner))
+}
+
+func EnumeratePackageVersions(ctx *context.Context) {
+	p, err := packages_model.GetPackageByName(ctx, ctx.Package.Owner.ID, packages_model.TypeCargo, ctx.Params("package"))
+	if err != nil {
+		if errors.Is(err, util.ErrNotExist) {
+			apiError(ctx, http.StatusNotFound, err)
+		} else {
+			apiError(ctx, http.StatusInternalServerError, err)
+		}
+		return
+	}
+
+	b, err := cargo_service.BuildPackageIndex(ctx, p)
+	if err != nil {
+		apiError(ctx, http.StatusInternalServerError, err)
+		return
+	}
+	if b == nil {
+		apiError(ctx, http.StatusNotFound, nil)
+		return
+	}
+
+	ctx.PlainTextBytes(http.StatusOK, b.Bytes())
+}
+
 type SearchResult struct {
 	Crates []*SearchResultCrate `json:"crates"`
 	Meta   SearchResultMeta     `json:"meta"`