pallet-sudo: add `CheckOnlySudoAccount` signed extension (#12496)

* pallet-sudo: add `CheckSudoKey` signed extension

Signed-off-by: koushiro <koushiro.cqx@gmail.com>

* Rename CheckSudoKey => CheckOnlySudo

Signed-off-by: koushiro <koushiro.cqx@gmail.com>

* Rename extension name and Add some docs

* Apply review suggestions

* Update frame/sudo/src/extension.rs

Co-authored-by: Jegor Sidorenko <5252494+jsidorenko@users.noreply.github.com>

* Update frame/sudo/src/extension.rs

Signed-off-by: koushiro <koushiro.cqx@gmail.com>
Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: Jegor Sidorenko <5252494+jsidorenko@users.noreply.github.com>