resolvectl: implement --json flag for resolvectl status

resolvectl: implement --json flag for resolvectl status

Add --json support for all status commands in resolvectl by making use
of the new DumpDNSConfiguration varlink method. E.g,

$ resolvectl --json=pretty status eth0
[
	{
		"ifname" : "eth0",
		"ifindex" : 9,
		"defaultRoute" : true,
		"currentServer" : {
                        "addressString" : "10.148.181.1",
			"address" : [
				10,
				148,
				181,
				1
			],
			"family" : 2,
			"port" : 53,
			"ifindex" : 9,
			"accessible" : true
		},
		"servers" : [
			{
                                "addressString" : "10.148.181.1",
				"address" : [
					10,
					148,
					181,
					1
				],
				"family" : 2,
				"port" : 53,
				"ifindex" : 9,
				"accessible" : true
			}
		],
		"searchDomains" : [
			{
				"name" : "local",
				"routeOnly" : false,
				"ifindex" : 9
			}
		],
		"dnssec" : "allow-downgrade",
		"dnsOverTLS" : "no",
		"llmnr" : "no",
		"mDNS" : "no",
		"scopes" : [
			{
				"protocol" : "dns",
				"ifindex" : 9,
				"ifname" : "eth0",
				"dnssec" : "allow-downgrade",
				"dnsOverTLS" : "no"
			}
		]
	}
]

Like the regular status output, fields are omitted all together when
empty, unless explicitly requested via one of the sub-commands dns,
domain, nta, etc.