paras: add governance control dispatchables (#4575)

* paras: add governance control dispatchables

Adds a couple of functions for governance control for the paras module
in the anticipation of PVF pre-checking enabling.

Specifically, this commit adds a function for pre-registering a PVF that
governance trusts enough. This function will come in handy in case there
is a parachain that does not follow the GoAhead signal. That is, does
not include https://github.com/paritytech/cumulus/pull/517.

This may be not an exhaustive list of the functions that may come in
handy. Any suggestions to add more are welcome.

* cargo run --quiet --release --features=runtime-benchmarks -- benchmark --chain=kusama-dev --steps=50 --repeat=20 --pallet=runtime_parachains::paras --extrinsic=* --execution=wasm --wasm-execution=compiled --heap-pages=4096 --header=./file_header.txt --output=./runtime/kusama/src/weights/runtime_parachains_paras.rs

* cargo run --quiet --release --features=runtime-benchmarks -- benchma...