[R] Add functionality to allow binding registration with the `pkg::fun` name

[asfimport]

• Modify register_binding() to register 2 identical copies for each pkg::fun binding, fun and pkg::fun.
• Add a binding for the :: operator, which helps with retrieving bindings from the function registry.
• Add generic unit tests for the pkg::fun functionality.

Reporter: Dragoș Moldovan-Grünfeld / @dragosmg
Assignee: Dragoș Moldovan-Grünfeld / @dragosmg

_{Note: This issue was originally created as ARROW-16888. Please see the migration documentation for further details.}