Feat: add stellar ledger as signer #469

Job	Run time
build (stellar-cli, stellar, ubuntu-20.04, x86_64-unknown-linux-gnu)	11m 53s
build (stellar-cli, stellar, ubuntu-20.04, aarch64-unknown-linux-gnu)	13m 54s
build (stellar-cli, stellar, macos-14, x86_64-apple-darwin)	9m 34s
build (stellar-cli, stellar, macos-12, aarch64-apple-darwin)	18m 50s
build (stellar-cli, stellar, windows-latest, x86_64-pc-windows-msvc, .exe)	20m 4s
build (soroban-cli, soroban, ubuntu-20.04, x86_64-unknown-linux-gnu)	12m 13s
build (soroban-cli, soroban, ubuntu-20.04, aarch64-unknown-linux-gnu)	12m 58s
build (soroban-cli, soroban, macos-14, x86_64-apple-darwin)	8m 50s
build (soroban-cli, soroban, macos-12, aarch64-apple-darwin)	14m 10s
build (soroban-cli, soroban, windows-latest, x86_64-pc-windows-msvc, .exe)	21m 33s
	2h 23m 59s

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