feat: `tx sign` and new signer #1135

Job	Run time
build (stellar-cli, stellar, ubuntu-20.04, x86_64-unknown-linux-gnu)	15m 21s
build (stellar-cli, stellar, ubuntu-20.04, aarch64-unknown-linux-gnu)	16m 20s
build (stellar-cli, stellar, macos-14, x86_64-apple-darwin)	11m 2s
build (stellar-cli, stellar, macos-12, aarch64-apple-darwin)	24m 49s
build (stellar-cli, stellar, windows-latest, x86_64-pc-windows-msvc, .exe)	21m 46s
build (soroban-cli, soroban, ubuntu-20.04, x86_64-unknown-linux-gnu)	14m 34s
build (soroban-cli, soroban, ubuntu-20.04, aarch64-unknown-linux-gnu)	17m 8s
build (soroban-cli, soroban, macos-14, x86_64-apple-darwin)	11m 6s
build (soroban-cli, soroban, macos-12, aarch64-apple-darwin)	22m 54s
build (soroban-cli, soroban, windows-latest, x86_64-pc-windows-msvc, .exe)	20m 14s
	2h 55m 14s

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