The struct for the /commit rpc response is currently different from the go-lang implementation. This could be on purpose: commits were or are about to be refactored (to consist of only signatures).
The light client will talk to the rpc endpoint exposed by tendermint. Hence, the types should now match the go implementation (or introduce a new compatible type exclusively for the light client if desired).

Current SignedHeader in Rust:
https://github.com/interchainio/tendermint-rs/blob/5844dc5c0faa537b4225c5221cd863285b97d8ce/tendermint/src/rpc/endpoint/commit.rs#L44-L47

SignedHeader in golang:
https://github.com/tendermint/tendermint/blob/51dc810d041eaac78320adc6d53ad8b160b06601/types/block.go#L708-L713

encapsulated in the ResultCommit response:
https://github.com/tendermint/tendermint/blob/51dc810d041eaac78320adc6d53ad8b160b06601/rpc/core/types/responses.go#L34-L37

by this endpoint:
https://github.com/tendermint/tendermint/blob/51dc810d041eaac78320adc6d53ad8b160b06601/rpc/core/blocks.go#L317

ref: #31 #36