Implement `cmd/exit sign`

[varex83]

Scope

Port charon/cmd/exit_sign.go (~330 LOC) — the pluto exit sign subcommand that produces a partial signature on a voluntary-exit message and pushes it to the obol-api.

References

• Go: https://github.com/ObolNetwork/charon/blob/main/cmd/exit_sign.go
• Tests: cmd/exit_sign_internal_test.go (~494 LOC)

Acceptance

• Flag/help parity with charon
• Tests ported and green
• cargo clippy --workspace --all-targets --all-features -- -D warnings clean