fix(block-producer): add request timeout to validator sign_block call

[SantiagoPittella]

Summary

Gives the validator request an explicit timeout so a dropped connection fails.

Changes:

• BlockProducerValidatorClient::new now takes a timeout and uses .with_timeout() instead of .without_timeout().
• New DEFAULT_VALIDATOR_TIMEOUT (30s).
• New --validator.timeout / MIDEN_NODE_VALIDATOR_TIMEOUT CLI flag

Changelog

[[entry]]
scope       = "node"
impact      = "fixed"
description = "Sequencer no longer halts block production for ~15 minutes when the validator connection drops; the `sign_block` call now times out and retries."

[[entry]]
scope       = "node"
impact      = "added"
description = "Added `--validator.timeout` / `MIDEN_NODE_VALIDATOR_TIMEOUT` to configure the request timeout for calls to the validator (default 30s)."