Make it possible to dynamically provide additional routes for entities

Problem/Motivation

To provide a serialization of an entity, the serialization module will have to add a new route for the entity. This route should have the same URI template as the original route, but a different format and possibly a different _method.

This route will map to a controller in the serialization module. This controller will have to load the entity.

The two pieces of information that the controller would need to perform an entity_load are the entity type and the entity id. The entity type should be possible to add as an argument to the route at the time of route definition. However, the position of the entity id placeholder in the URI template is not available to the serialization module.

While one possibility might be to just include the full route string in the URI callback, that would contain the entity-specific variable name for the ID, such as 'nid'. Because Symfony's wildcard placeholder matches on variable name, there would have to be a different controller for each variable name (i.e. one for nid, one for cid, etc).

Proposed resolution

There are a couple of possible solutions:

1. Provide an 'entity_id argument' for the path, similar to the 'bundle argument' that is provided in the admin array.
2. Use a route such as entity/{entity_type}/{eid} as the canonical URI for all entities. Then, alias (node|user|comment)/{id} to that route.