feat: `tx sign` and new signer #1143

Job	Run time
build (stellar-cli, stellar, ubuntu-20.04, x86_64-unknown-linux-gnu)	14m 45s
build (stellar-cli, stellar, ubuntu-20.04, aarch64-unknown-linux-gnu)	16m 32s
build (stellar-cli, stellar, macos-14, x86_64-apple-darwin)	11m 31s
build (stellar-cli, stellar, macos-12, aarch64-apple-darwin)	19m 48s
build (stellar-cli, stellar, windows-latest, x86_64-pc-windows-msvc, .exe)	21m 40s
build (soroban-cli, soroban, ubuntu-20.04, x86_64-unknown-linux-gnu)	14m 51s
build (soroban-cli, soroban, ubuntu-20.04, aarch64-unknown-linux-gnu)	17m 33s
build (soroban-cli, soroban, macos-14, x86_64-apple-darwin)	10m 34s
build (soroban-cli, soroban, macos-12, aarch64-apple-darwin)	22m 29s
build (soroban-cli, soroban, windows-latest, x86_64-pc-windows-msvc, .exe)	21m 58s
	2h 51m 41s

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