reverseproxy: Add placeholder for networkAddr in active health check headers

[dylanschultzie]

Follows: #6440

Previously hostAddr would duplicate the Host header. This now appropriately passes the full original upstream:

5.0.0.33 - - [09/Jul/2024 15:35:04] "GET /health HTTP/1.1" 200 -
+Host: 5.0.0.33:53336
User-Agent: Go-http-client/1.1
+Full-Upstream: 5.0.0.33:8545
Accept-Encoding: gzip

Resolves forum post: https://caddy.community/t/can-you-pass-in-the-full-upstream-to-an-active-health-check/24718

[CLAassistant]

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