ExecIr: OpenExec implementation of invertible rigs

The execIr library is built on top of execUsd.

[!note] The schemas and APIs defined in this library are currently limited in functionality and not intended for production use.

The execIr library contains utilities that are used to define invertible controllers. This library also contains code used to implement the execution behaviors for invertible rigging contrllers, including basic controller types that are used as building blocks for other, higher-level controllers.