This ensures that the X-Grafana-User header can be trusted.
If the configuration enabled the setting of this header, the
server can now trust that X-Grafana-User is set/unset by Grafana.
Before this, an anonymous user could simply set the X-Grafana-User
header themselves (using the developer tool for example)
This adds support for using templated/dynamic urls in routes.
* refactor interpolateString into utils and add interpolation support for app plugin routes.
* cleanup and add error check for url parse failure
* add docs for interpolated route urls
Closes#16835
