Implement `cmd/exit broadcast`

[varex83]

Scope

Port charon/cmd/exit_broadcast.go (~335 LOC) — the pluto exit broadcast subcommand that submits a previously-aggregated voluntary exit to the beacon network.

References

• Go: https://github.com/ObolNetwork/charon/blob/main/cmd/exit_broadcast.go
• Tests: cmd/exit_broadcast_internal_test.go (~544 LOC)

Acceptance

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