Add support for namespaces

[LykaiosNZ]

• Adds Namespace field to definitions. This is always nullable even when a definition has a namespace, for consistency.
• Adds includeNamespace optional parameter to WithId (defaults to false):

public const string Name = "user";
public const string? Namespace = "mynamespace";
public static string WithId(string id, bool includeNamespace = false) => includeNamespace ? $"mynamespace/user:{id}" : $"user:{id}";

The includeNamespace parameter is available even when the definition does not have a namespace for consistency, however the implementation will ignore this:

public const string Name = "user";
public const string? Namespace = null;
public static string WithId(string id, bool includeNamespace = false) => $"user:{id}";

Closes #24

+semver:minor