recipetool: create: add new optional process_url callback for plugins

recipetool: create: add new optional process_url callback for plugins

Add a new process_url callback that plugins can optionally implement if
they which to handle url.

Plugins can implement this callback for example, to:
* transform the url
* add special variables using extravalues
* add extra classes
* ...

If a plugin handles the url, it must append 'url' to the handled
list and must return the fetchuri

No functional changes expected for plugins non implementing this
optional callback

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>