feat(reverse_proxy): lb_retry_on conditions

[seroperson]

Closes #6267

Added a new field, lb_retry_on which is responsible for retrying based on response.

Available conditions:

• Status codes: 502, 503, 5xx-like
• empty_response - socket closed before receiving a response
• tls_handshake_error - TLS negotiation failed
• response_timeout - upstream was too slow to respond
• other - any other unclassified error

In Caddyfile it looks like:

reverse_proxy backend1:8080 backend2:8080 {
    lb_retries 3
    lb_retry_on 503 502 response_timeout
}

If lb_retry_on is unset, the old default behavior is preserved if retry-related options are set:

1. Dial errors (connection refused, DNS failure, etc) - always retried
2. All other errors (EOF, timeout, TLS errors, etc):
   2.1 GET requests - always retried
   2.2 Non-GET requests - retried only if lb_retry_match is configured and the request matches it
3. Status codes - never trigger retries

Now lb_retry_on allows you to mostly override the default behavior (except of dial errors, they're always retried).

Assistance Disclosure

Co-Authored with Claude Opus 4.6, mostly for tests and consulting about the codebase.

[CLAassistant]

All committers have signed the CLA.