contracts: Composable `ChainExtension` (#11816)

* Add `RegisteredChainExtension`

* Add tests

* Update frame/contracts/src/chain_extension.rs

Co-authored-by: Michael Müller <[email protected]>

* Add more docs

* Remove debugging leftover

* Make ChainExtension-registry lowercase

* Apply suggestions from code review

Co-authored-by: Hernando Castano <[email protected]>

* Improve clarity of test inputs

Co-authored-by: Michael Müller <[email protected]>
Co-authored-by: Hernando Castano <[email protected]>