GRAILS-11796: Add decorators to the GSP source

https://jira.grails.org/browse/GRAILS-11796

Adds a new `GroovyPagesTemplateEngine.setGroovyPageSourceDecorators(List<GroovyPageSourceDecorator>)` method that allows for the decoration of the source of the GSP.

To use this functionality, I imagine plugins or applications (via BootStrap.groovy or some equivalent mechanism) can call:
```
groovyPagesTemplateEngine.getGroovyPageSourceDecorators().add(myDecorator)
```
or, in a plugin's "doWithSpring", they could do:
```
def groovyPagesTemplateEngine = delegate.getBeanDefinition("groovyPagesTemplateEngine")
groovyPagesTemplateEngine.getPropertyValue("groovyPageSourceDecorators").add(myDecorator)
```