Access log vars

[nitrag]

Resolves #312

Adds the ability to implement UserAgent and custom request/response headers to the access log.

• Add user_agent access log variable for logging the request User-Agent header
• Add support for arbitrary request headers via %({Header-Name}i)s syntax (gunicorn-compatible)
• Add support for arbitrary response headers via %({Header-Name}o)s syntax (gunicorn-compatible)
• Validate access log format strings eagerly at startup to catch misconfiguration early
• Response header capture overhead is gated behind a needs_resp_headers flag — zero cost when no %({...}o)s atoms are in the format string

[nitrag]

@gi0baro ready for review, thanks