caddyhttp: add http.request.local{,.host,.port} placeholder (#6182)

* caddyhttp: add `http.request.local{,.host,.port}` placeholder

This is the counterpart of `http.request.remote{,.host,.port}`.

`http.request.remote` operates on the remote client's address, while
`http.request.local` operates on the address the connection arrived on.

Take the following example:

- Caddy serving on `203.0.113.1:80`
- Client on `203.0.113.2`

`http.request.remote.host` would return `203.0.113.2` (client IP)

`http.request.local.host` would return `203.0.113.1` (server IP)
`http.request.local.port` would return `80` (server port)

I find this helpful for debugging setups with multiple servers and/or
multiple network paths (multiple IPs, AnyIP, Anycast).

Co-authored-by: networkException <git@nwex.de>

* caddyhttp: add unit test for `http.request.local{,.host,.port}`

* caddyhttp: add integration test for `http.request.local.port`

* caddyhttp: fix `http.request.local.host` placeholder handling with unix sockets

The implementation matches the one of `http.request.remote.host` now and
returns the unix socket path (just like `http.request.local` already did)
instead of an empty string.

---------

Co-authored-by: networkException <git@nwex.de>

modules/caddyhttp/app.go

@@ -68,6 +68,9 @@ func init() {
 // `{http.request.orig_uri.query}` | The request's original query string (without `?`)
 // `{http.request.port}` | The port part of the request's Host header
 // `{http.request.proto}` | The protocol of the request
+// `{http.request.local.host}` | The host (IP) part of the local address the connection arrived on
+// `{http.request.local.port}` | The port part of the local address the connection arrived on
+// `{http.request.local}` | The local address the connection arrived on
 // `{http.request.remote.host}` | The host (IP) part of the remote client's address
 // `{http.request.remote.port}` | The port part of the remote client's address
 // `{http.request.remote}` | The address of the remote client