requestbody: Add Caddyfile support (#3859)

* Add Caddyfile support for request_body:

```
  request_body {
    max_size 10000000
  }
```

* Improve Caddyfile parser for request_body module

* Remove unnecessary `continue`

* Add sample for caddyfile_adapt_test

modules/caddyhttp/requestbody/caddyfile.go

@@ -0,0 +1,51 @@
+// Copyright 2015 Matthew Holt and The Caddy Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package requestbody
+
+import (
+	"github.com/caddyserver/caddy/v2/caddyconfig/httpcaddyfile"
+	"github.com/caddyserver/caddy/v2/modules/caddyhttp"
+	"github.com/dustin/go-humanize"
+)
+
+func init() {
+	httpcaddyfile.RegisterHandlerDirective("request_body", parseCaddyfile)
+}
+
+func parseCaddyfile(h httpcaddyfile.Helper) (caddyhttp.MiddlewareHandler, error) {
+	rb := new(RequestBody)
+
+	for h.Next() {
+		// configuration should be in a block
+		for h.NextBlock(0) {
+			switch h.Val() {
+			case "max_size":
+				var sizeStr string
+				if !h.AllArgs(&sizeStr) {
+					return nil, h.ArgErr()
+				}
+				size, err := humanize.ParseBytes(sizeStr)
+				if err != nil {
+					return nil, h.Errf("parsing max_size: %v", err)
+				}
+				rb.MaxSize = int64(size)
+			default:
+				return nil, h.Errf("unrecognized servers option '%s'", h.Val())
+			}
+		}
+	}
+
+	return rb, nil
+}