Implement query_header_at_height()

[ancazamfir]

Crate

relayer

Summary

Implement https://github.com/informalsystems/ibc-rs/blob/ab6cac1e3dec846b9fb5c2fee0deb6a634f0f290/relayer/src/chain.rs#L112

Problem Definition

Missing implementation. To build a MsgCreateClient the relayer needs a recent LigthtBlock. For MsgUpdateClient we would need the get_minimal_set() but the query_header_at_height() above would be great if it's faster.

Proposal

• Initially, retrieve the header using plain RPC calls
• At a later stage, retrieve and verify the header via the light client

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[ancazamfir]

Closed via #364 (see build_header() in chain.rs)