Ability to configure arbitrary EL variables

[adrienbrault]

WDYT of being able to configure EL variables, that would be a bit like twig global variables:

bazinga_hateoas:
    expression_language:
        context_variables:
            base_rel: https://rels.example.com
            foo: bar

To then be able to:

/**
 * @Hateoas\Relation("expr(base_rel ~ '/users')", ...)
 */

[willdurand]

Yeah! Fairly easy.

[nyedidikeke]

If #24 is a duplicate of #13 and is closed, then we should consider closing Ability to configure arbitrary EL variables #13 (the current issue) as well.