Implement `cmd/exit fetch`

[varex83]

Scope

Port charon/cmd/exit_fetch.go (~185 LOC) — the pluto exit fetch subcommand that downloads aggregated partial exits from the obol-api.

References

• Go: https://github.com/ObolNetwork/charon/blob/main/cmd/exit_fetch.go
• Tests: cmd/exit_fetch_internal_test.go (~366 LOC)

Acceptance

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