Snowbridge Beacon header age check and add linear fee multiplier to ensure safety margins (#3791)

This is a cherry-pick from master of
https://github.com/paritytech/polkadot-sdk/pull/3727 and
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