expose add_proxy and remove_proxy (#7189)

* expose add_proxy and remove_proxy

* add getter

* bump version

* Update frame/proxy/src/lib.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* add comments

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>