Light client extrinsic root mismatch

[bkchr]

I pushed all my changes. The changes to substrate are trivial:

• making service/chain_spec modules pub
• enable mdns on dev chains so that the light client can discover the node
• disable root checking in the fetcher
• try getting blocks to finalize by setting justification_period to 1, but that doesn't seem to work

To reproduce (assumes the substrate/substrate-subxt are side by side):

cd substrate-subxt/light-client
./gen-chain-spec.sh
./run.sh
cargo test -- test_light_client --ignored

• [0] Support embedded full/light node clients. subxt#91
• [1] https://github.com/dvc94ch/substrate/tree/light-client

Originally posted by @dvc94ch in #6143 (comment)

[bkchr]

CC @svyatonik maybe you got some time to look into this? The linked issue contains some more discussion around the problem.