Add attribute flag building and parsing (#1562)

This adds a few command sketches but that's not the main point of the PR
and filling those out will come in a different PR. This adds a new flag
type that can combine inputs to a single target slice, retaining the
ability to distinguish where it came from but also keep ordering across
different flag names. It also implements a function that does the
heuristic parsing with fallback.

internal/cmd/base/base.go

@@ -83,6 +83,10 @@ type Command struct {
 	FlagRecursive         bool
 	FlagFilter            string
 
+	// Attribute values
+	FlagAttributes string
+	FlagAttrs      []CombinedSliceFlagValue
+
 	client *api.Client
 }