Implement `cmd/addvalidators`

[varex83]

Scope

Port charon/cmd/addvalidators.go (~255 LOC) — the pluto addvalidators command that adds validators to an existing cluster.

References

• Go: https://github.com/ObolNetwork/charon/blob/main/cmd/addvalidators.go
• Tests: cmd/addvalidators_internal_test.go (~441 LOC)

Acceptance

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