feat: `tx sign` and new signer #1195

Job	Run time
build (stellar-cli, stellar, ubuntu-20.04, x86_64-unknown-linux-gnu)	15m 55s
build (stellar-cli, stellar, ubuntu-20.04, aarch64-unknown-linux-gnu)	15m 58s
build (stellar-cli, stellar, macos-14, x86_64-apple-darwin)	9m 30s
build (stellar-cli, stellar, macos-12, aarch64-apple-darwin)	18m 5s
build (stellar-cli, stellar, windows-latest, x86_64-pc-windows-msvc, .exe)	20m 26s
build (soroban-cli, soroban, ubuntu-20.04, x86_64-unknown-linux-gnu)	14m 27s
build (soroban-cli, soroban, ubuntu-20.04, aarch64-unknown-linux-gnu)	15m 47s
build (soroban-cli, soroban, macos-14, x86_64-apple-darwin)	11m 2s
build (soroban-cli, soroban, macos-12, aarch64-apple-darwin)	15m 59s
build (soroban-cli, soroban, windows-latest, x86_64-pc-windows-msvc, .exe)	22m 34s
	2h 39m 43s

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