[Twig Hooks] No @= support in the configuration

[jakubtobiasz]

We can access the configuration and context bags in the context (YAML configuration) with using @=.

Example:

twig_hooks:
	hooks:
		'my_hook':
			template: 'my_template.html.twig'
			context:
				some_key: '@=_context.something'

It will retrieve the value of the something key from the context. The same applies for the _configuration.

Sometimes, we might need to access the context or configuration bag in the configuration.

twig_hooks:
	hooks:
		'my_hook':
			template: 'my_template.html.twig'
			configuration:
				attribute:
					'channel-id': '@=_context.resource.id'

The goal of this issue is to reuse the logic behind the bags resolving while defining the context in the configuration declaration.