[TwigBundle] Remove hard dependency of RequestContext in AssetsExtension

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11466
| License       | MIT
| Doc PR        | -

symfony/symfony#10451 introduced the requirement of RequestContext in AssetsExtension. This is only needed when generating absolute URLs for assets. When sending emails with Twig from the CLI, the router may not be present and most times is not required. 

This PR attempts to remove the hard dependency and set RequestContext if the router is present according to issue #11466.
This message looks weird to me. "To ensure a URL as absolute..." -> "To generate an absolut URL for an asset, ..."