feat(pbs): let a mux override proposer_deadline_buffer_ms

[JasonVranek]

Builds on #506.

A [[mux]] can now set proposer_deadline_buffer_ms for the keys it lists, as it already can timeout_get_header_ms and late_in_slot_time_ms. A key outside every mux keeps the [pbs] value. The mux's value has the same limits as [pbs]: above 0 and under one slot, checked when the config loads.

[[mux]]
id = "remote-vcs"
validator_pubkeys = ["0x..."]
proposer_deadline_buffer_ms = 200

[[mux.relays]]
id = "relay-a"
url = "https://0x...@relay-a.example.com"

Changes

• MuxConfig takes an optional proposer_deadline_buffer_ms, filled from [pbs] when unset, like the two existing overrides.
• A mux that fails its config checks is named in the error (mux <id>: ...), and the warning for a bid request with no time left after the buffer names the buffer and the mux.
• Docs: the ePBS page (including the buffer's troubleshooting row, which now covers a mux), the mux key loaders page (which also said a mux affects only get_header) and config.example.toml.
• The mux test helper builds its mux through validate_and_fill, so the override and its limits run as they do in production. wait_for_ready gets back the doc comment that feat(pbs): dial builders outside the config for ePBS requests #506 left on MuxedRelays.