glutton: also increase parachain block length (#4728)

Glutton currently is useful mostly for stress testing relay chain
validators. It is unusable for testing the collator networking and block
announcement and import scenarios. This PR resolves that by improving
glutton pallet to also buff up the blocks, up to the runtime configured
`BlockLength`.

### How it works
Includes an additional inherent in each parachain block. The `garbage`
argument passed to the inherent is filled with trash data. It's size is
computed by applying the newly introduced `block_length` percentage to
the maximum block length for mandatory dispatch class. After
https://github.com/paritytech/polkadot-sdk/pull/4765 is merged, the
length of inherent extrinsic will be added to the total block proof
size.

The remaining weight is burnt in `on_idle` as configured by the
`storage` percentage parameter.


TODO:
- [x] PRDoc
- [x] Readme update
- [x] Add tests

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Signed-off-by: Andrei Sandu <[email protected]>