feat(pbs): a switch to turn off dialing, and a 20 ms ePBS buffer default

[JasonVranek]

Summary

Two ePBS config follow-ups to #506:

1. A [pbs] dial_unconfigured_builders switch that turns off dialing builders outside the config, from the review on dial.rs.
2. proposer_deadline_buffer_ms defaults to 20 instead of 50, and setup no longer asks for it as the first config item to change.

Dial switch

Since #506, an ePBS bid or builder preferences request whose auth_data matches none of the key's relay entries in the Commit-Boost config is dialed at the builder that auth_data specifies. dial_unconfigured_builders lets an operator turn that off and keep Commit-Boost only to the builders in its config.

[pbs]
dial_unconfigured_builders = false

The default is true, which keeps #506's behaviour. The switch applies to the whole instance - there's no per-mux override.

When it is off:

• Configured relays: mux relays and [[relays]] serve requests exactly as before.
• Anything else: a request whose auth_data doesn't match any of the key's relay entries gets 400 "auth.message.data does not match any configured builder". Commit-Boost logs a warning with the auth_data.
• Order of checks: the switch is checked before the chained-Commit-Boost and own-host checks. Those also answer 400, so the only difference is the log line.

Buffer default

Commit-Boost's own share of the buffer is a few milliseconds on a release build. Measured with a relay that never answers, it used 1.8 ms p50, 2.9 ms p99 and 3.7 ms at most, over 3000 requests. About 2 ms of that is millisecond rounding.

So 20 suits a beacon node on the same host. The setting stays for a beacon node on another host, where the response's trip back and any clock difference between the hosts come out of the buffer.