Add a description slot to RadioButton
Summary
This change upstreams the inclusion of the description slot for RadioButton.vue.
This slot was a customization used by Rancher Desktop to provide arbitrary content to a radio button's description. For example, using code blocks in the description:

descriptionKey & description props should take priority over the slot in order to preserve backward compatibility.
Areas or cases that should be tested
We need to look out for regressions in RadioButton usage. Performing a search for RadioButton should provide a wide array of examples to test. Since this slot is optional, there should be no change to the existing radio button implementations. Any observed change in output is to be considered a regression.
closes #6649
Signed-off-by: Phillip Rak [email protected]