Add weight of our signed extensions to the messages pallet calls

[svyatonik]

So bridge hub will have our signed extensions that reject obsolete transactions and pay reward/slash relayers. The obsolete-check reads the same storage entries as the call itself does, so we assume it brings no additional weight (it does, but not that much). However, the post_dispatch may add/update entries of the pallet-bridge-relayers rewards map. So we should add this weight to our call weights (delivery and confirmation). Maybe something as simple as DbWeight.reads_writes(1, 1).set_proof_size(<proof-size-that-we-may-get-from-relayers-pallet-benchmarks>). But maybe some benchmark is ok too.