Snowbridge - add a linear fee multiplier to ensure safety margins (#3794)

This is a cherry-pick from master of
https://github.com/paritytech/polkadot-sdk/pull/3790



Expected patches for (1.7.0):
snowbridge-pallet-ethereum-client
snowbridge-pallet-inbound-queue
snowbridge-pallet-outbound-queue
snowbridge-outbound-queue-runtime-api
snowbridge-pallet-system
snowbridge-core

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Co-authored-by: Vincent Geddes <[email protected]>
Co-authored-by: claravanstaden <Cats 4 life!>
Co-authored-by: Vincent Geddes <[email protected]>
Co-authored-by: Adrian Catangiu <[email protected]>