caddyfile: Add support for args on imports

[francislavoie]

Stems from the discussion in https://caddy.community/t/logging-snippet-using-host-placeholder-for-log-filename/8218

The idea here is that it's common for users to want to have one element of an imported snippet or file be replaced with a context-specific value.

This PR adds support for {args.*} placeholders at Caddyfile-parse time when importing tokens from either a snippet or file, where * is the positional argument number.

The import syntax becomes:

import <pattern> <args...>

---

For example, consider this Caddyfile, where a user wants to enable logging using a common config for each of their sites. You'll see that the log filename is expanded to /var/log/caddy/a.example.com.access.log:

(logging) {
  log {
    output file /var/log/caddy/{args.0}.access.log
  }
}

a.example.com {
  import logging a.example.com
}

b.example.com {
  import logging b.example.com
}

Adapted JSON output:

{
	"logging": {
		"logs": {
			"default": {
				"exclude": [
					"http.log.access.log0",
					"http.log.access.log1"
				]
			},
			"log0": {
				"writer": {
					"filename": "/var/log/caddy/a.example.com.access.log",
					"output": "file"
				},
				"include": [
					"http.log.access.log0"
				]
			},
			"log1": {
				"writer": {
					"filename": "/var/log/caddy/b.example.com.access.log",
					"output": "file"
				},
				"include": [
					"http.log.access.log1"
				]
			}
		}
	},
	"apps": {
		"http": {
			"servers": {
				"srv0": {
					"listen": [
						":443"
					],
					"routes": [
						{
							"match": [
								{
									"host": [
										"a.example.com"
									]
								}
							],
							"terminal": true
						},
						{
							"match": [
								{
									"host": [
										"b.example.com"
									]
								}
							],
							"terminal": true
						}
					],
					"logs": {
						"logger_names": {
							"a.example.com": "log0",
							"b.example.com": "log1"
						}
					}
				}
			}
		}
	}
}

---

This also supports an arbitrary number of arguments, and supports imported files.

The file respond.txt

respond "I am {args.0}, hears {args.1}"

Caddyfile:

example.com

import respond.txt Groot Rocket
import respond.txt you "the confused man"

Adapted JSON:

{
	"apps": {
		"http": {
			"servers": {
				"srv0": {
					"listen": [
						":443"
					],
					"routes": [
						{
							"match": [
								{
									"host": [
										"example.com"
									]
								}
							],
							"handle": [
								{
									"handler": "subroute",
									"routes": [
										{
											"handle": [
												{
													"body": "I am groot, hears rocket",
													"handler": "static_response"
												},
												{
													"body": "I am you, hears the confused man",
													"handler": "static_response"
												}
											]
										}
									]
								}
							],
							"terminal": true
						}
					]
				}
			}
		}
	}
}

[sumnerboy12]

Looks great @francislavoie, thanks!

[mholt]

LGTM, we can give this a try. Thank you!