archive: add CompressLevel type and consts (MR 25)

There's really not a great way to map individual levels to each
compression library, so this just adds a new type that will invoke the
three relevant levels for each library used. This could be improved in
the future.

internal/archive/archive.go

@@ -29,6 +29,17 @@ const (
 	FormatZstd CompressFormat = "zstd"
 )
 
+type CompressLevel string
+
+const (
+	// Mapped to the "default" level for the given format
+	LevelDefault CompressLevel = "default"
+	// Maps to the fastest compression level for the given format
+	LevelFast CompressLevel = "fast"
+	// Maps to the best compression level for the given format
+	LevelBest CompressLevel = "best"
+)
+
 type Archive struct {
 	items           archiveItems
 	cpioWriter      *cpio.Writer