Configurable resource file extensions eligible for EL evaluation

At the moment only CSS resources are allowed to contain EL expressions that will be evaluated when served as a resource. An application my serve other resources that could contain dynamic content. The spec should allow configuring file extensions eligible for EL evaluation when served as resources.
#### Affected Versions
[2.2]

Start by reviewing the Jakarta Faces resource-serving specification and the existing rule that permits EL evaluation only for CSS resources. Define how applications configure eligible file extensions and what resource-serving behavior should result. Done means the specification clearly permits configurable extensions for dynamic resource content.

Written by the indexing model from the issue text.