Let a requirement be optional, binding only a provider that already runs (hq issue 166, ADR 0330)

Making every soft dependency a provision made it hard: a mesh without influxdb could not assign
grafana, and one with influxdb in its catalogue would install it for grafana. A requirement may now
be written {"name": ..., "optional": true}. It resolves as a hard one when a provider is assigned or
reachable, and is otherwise absent: never refused, nothing installed for it. Its binding, credential
and only-when-bound files are left out, its ${bound:...|default:...} placeholders take their
defaults, the module check refuses one that could render empty, and plan and node list it unbound.

cmd/mesh-controller/modules.go

@@ -237,7 +237,15 @@ func moduleCommand(ctx context.Context, args []string) error {
 				says = append(says, "provides "+describeOffers(m.Provides))
 			}
 			if len(m.Requires) > 0 {
-				says = append(says, "requires "+strings.Join(m.Requires, ", "))
+				// An optional requirement says so (novox/hq ADR 0330): it is not refused when absent.
+				required := make([]string, 0, len(m.Requires))
+				for _, r := range m.Requires {
+					if m.IsOptional(r) {
+						r += " (optional)"
+					}
+					required = append(required, r)
+				}
+				says = append(says, "requires "+strings.Join(required, ", "))
 			}
 			for _, c := range m.Claims {
 				says = append(says, "claims "+c.At()+"/"+c.Name)