feat: deb (Debian/apt) local and remote repository support

Adds a deb provider with feature parity to rpm for local and remote repos,
so .deb packages can be hosted (flat apt repo generated on upload) and a
Debian/Ubuntu mirror can be cached through the proxy.

- add PackageDeb to the package-type enum
- new internal/provider/deb: Classify (.deb immutable, Packages/Release/dists
  mutable), ValidateUpload to pool/<file>, pure-Go .deb parse (ar -> control.tar
  gz/xz/zst -> ./control), and a flat-repo LocalIndexer serving Packages,
  Packages.gz and an unsigned Release ([trusted=yes], mirroring rpm gpgcheck=0)
- remote proxy path: UpstreamURL/ContentType/AuthHeaders via BasicHeaders
- DebMetadata struct + store interfaces on provider; deb_metadata table in
  migrate() and Insert/Delete/List DB methods
- blank-import the deb provider in the server
- testsupport.MinimalDeb pure-Go fixture builder
- unit tests (classify, control parse across gz/xz/zst, Packages/Release
  generation, validate) and a dockere2e TestLocalDebRepo

internal/testsupport/deb.go

@@ -0,0 +1,61 @@
+package testsupport
+
+import (
+	"archive/tar"
+	"bytes"
+	"compress/gzip"
+	"fmt"
+)
+
+// MinimalDeb builds a valid-enough Debian package in pure Go (no committed
+// binary fixture, no dpkg-deb): an ar archive of debian-binary, a gzip
+// control.tar.gz carrying ./control, and an (empty) gzip data.tar.gz. It is the
+// deb analog of MinimalRPM and is parseable by the deb provider.
+func MinimalDeb(name, version, arch string) []byte {
+	control := fmt.Sprintf(
+		"Package: %s\nVersion: %s\nArchitecture: %s\nMaintainer: e2e <e2e@example.com>\n"+
+			"Section: utils\nPriority: optional\nDescription: minimal test package\n",
+		name, version, arch)
+
+	controlTarGz := gzipBytes(tarSingle("./control", []byte(control)))
+	dataTarGz := gzipBytes(tarEmpty())
+
+	var buf bytes.Buffer
+	buf.WriteString("!<arch>\n")
+	arWrite(&buf, "debian-binary", []byte("2.0\n"))
+	arWrite(&buf, "control.tar.gz", controlTarGz)
+	arWrite(&buf, "data.tar.gz", dataTarGz)
+	return buf.Bytes()
+}
+
+func arWrite(buf *bytes.Buffer, name string, data []byte) {
+	fmt.Fprintf(buf, "%-16s%-12s%-6s%-6s%-8s%-10d`\n", name, "0", "0", "0", "100644", len(data))
+	buf.Write(data)
+	if len(data)%2 == 1 {
+		buf.WriteByte('\n')
+	}
+}
+
+func tarSingle(name string, data []byte) []byte {
+	var buf bytes.Buffer
+	tw := tar.NewWriter(&buf)
+	tw.WriteHeader(&tar.Header{Name: name, Mode: 0o644, Size: int64(len(data)), Typeflag: tar.TypeReg})
+	tw.Write(data)
+	tw.Close()
+	return buf.Bytes()
+}
+
+func tarEmpty() []byte {
+	var buf bytes.Buffer
+	tw := tar.NewWriter(&buf)
+	tw.Close()
+	return buf.Bytes()
+}
+
+func gzipBytes(data []byte) []byte {
+	var buf bytes.Buffer
+	gz := gzip.NewWriter(&buf)
+	gz.Write(data)
+	gz.Close()
+	return buf.Bytes()
+}