feat: `tx sign` and new signer #1122

Job	Run time
build (stellar-cli, stellar, ubuntu-20.04, x86_64-unknown-linux-gnu)	15m 25s
build (stellar-cli, stellar, ubuntu-20.04, aarch64-unknown-linux-gnu)	16m 14s
build (stellar-cli, stellar, macos-14, x86_64-apple-darwin)	9m 16s
build (stellar-cli, stellar, macos-12, aarch64-apple-darwin)	18m 30s
build (stellar-cli, stellar, windows-latest, x86_64-pc-windows-msvc, .exe)	21m 43s
build (soroban-cli, soroban, ubuntu-20.04, x86_64-unknown-linux-gnu)	14m 31s
build (soroban-cli, soroban, ubuntu-20.04, aarch64-unknown-linux-gnu)	16m 35s
build (soroban-cli, soroban, macos-14, x86_64-apple-darwin)	12m 51s
build (soroban-cli, soroban, macos-12, aarch64-apple-darwin)	26m 41s
build (soroban-cli, soroban, windows-latest, x86_64-pc-windows-msvc, .exe)	22m 7s
	2h 53m 53s

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