Investigate supporting calculated hrefs in dynamic plugin nav extensions

In some cases, dynamic plugins may need to calculate a dynamic value for a nav extension href property. We should investigate ways to provide this capability. Some initial ideas include:

• href property accepts CodeRef callback
• Use well-known parameters similar to react-router routes and console will parse:
  • e.g. "/ns/:namespace/foo/bar" -> "/ns/active-namespace/foo/bar"