MINOR: acme: add 'dns-timeout' keyword for dns-01 challenge

MINOR: acme: add 'dns-timeout' keyword for dns-01 challenge

When using the dns-01 challenge method with "challenge-ready dns", HAProxy
retries DNS resolution indefinitely at the interval set by "dns-delay". This
adds a "dns-timeout" keyword to set a maximum duration for the DNS check phase
(default: 600s). If the next resolution attempt would be scheduled beyond that
deadline, the renewal is aborted with an explicit error message.

A new "dnsstarttime" field is stored in the acme_ctx to record when DNS
resolution began, used to evaluate the timeout on each retry.