[pallet-balances] `burn_allow_death` extrinsic (#3964)

Adds an additional extrinsic call to the `pallet-balances` to _burn_
tokens.
Depending on the `keep_alive` flag, the call might or might not reap the
account.

Required modification of the _fungible's_ `Mutate` trait, `burn_from`
function to allow the `Preservation` argument.

**TODO**
- [x] run benchmarks & update weights
- [x] make sure prdoc is required & properly formatted

Related issue: https://github.com/paritytech/polkadot-sdk/issues/3943

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Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: command-bot <>