Use frame umbrella crate in `pallet-proxy` and `pallet-multisig` (#5995)

A step towards https://github.com/paritytech/polkadot-sdk/issues/4782

In order to nail down the right preludes in `polkadot-sdk-frame`, we
need to migrate a number of pallets to be written with it. Moreover,
migrating our pallets to this simpler patter will encourage the
ecosystem to also follow along.

If this PR is approved and has no unwanted negative consequences, I will
make a tracking issue to migrate all pallets to this umbrella crate.

TODO: 

- [x] fix frame benchmarking template. Can we detect the umbrella crate
in there and have an `if else`? cc @ggwpez


- [x] Migrate benchmarking to v2 @re-gius a good candidate for you, you
can open a PR against my branch.
- [x] tracking issue with follow-ups

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Co-authored-by: Guillaume Thiolliere <[email protected]>
Co-authored-by: Giuseppe Re <[email protected]>
Co-authored-by: Dónal Murray <[email protected]>
Co-authored-by: Shawn Tabrizi <[email protected]>
Co-authored-by: Oliver Tale-Yazdi <[email protected]>