caddyfile: Pass blocks to import for snippets (#6130)

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caddytest/integration/caddyfile_adapt/import_block_snippet.caddyfiletest

@@ -0,0 +1,58 @@
+(snippet) {
+	header {
+		{block}
+	}
+}
+
+example.com {
+	import snippet {
+		foo bar
+	}
+}
+----------
+{
+	"apps": {
+		"http": {
+			"servers": {
+				"srv0": {
+					"listen": [
+						":443"
+					],
+					"routes": [
+						{
+							"match": [
+								{
+									"host": [
+										"example.com"
+									]
+								}
+							],
+							"handle": [
+								{
+									"handler": "subroute",
+									"routes": [
+										{
+											"handle": [
+												{
+													"handler": "headers",
+													"response": {
+														"set": {
+															"Foo": [
+																"bar"
+															]
+														}
+													}
+												}
+											]
+										}
+									]
+								}
+							],
+							"terminal": true
+						}
+					]
+				}
+			}
+		}
+	}
+}