Allow Link and LinkGenerator::generate() to provide attributes.

Problem/Motivation

Follow-up to #2614628: menu local task and local actions are missing link context variables

Link currently builds up attributes from the Url object. This is counter intuitive since the Url doesn't have attributes, the link does.

Proposed resolution

Let the Link and link generator deal with attributes. Leave the Url options attributes in for BC.

Remaining tasks

Needs tests.

API changes

Link objects can take

Data model changes