encode,staticfiles: Content negotiation, precompressed files (#4045)

* encode: implement prefer setting

* encode: minimum_length configurable via caddyfile

* encode: configurable content-types which to encode

* file_server: support precompressed files

* encode: use ReponseMatcher for conditional encoding of content

* linting error & documentation of encode.PrecompressedOrder

* encode: allow just one response matcher

also change the namespace of the encoders back, I accidently changed to precompressed >.>
default matchers include a *  to match to any charset, that may be appended

* rounding of the PR

* added integration tests for new caddyfile directives
* improved various doc strings (punctuation and typos)
* added json tag for file_server precompress order and encode matcher

* file_server: add vary header, remove accept-ranges when serving precompressed files

* encode: move Suffix implementation to precompressed modules

caddytest/integration/caddyfile_adapt_test.go

@@ -1,7 +1,10 @@
 package integration
 
 import (
+	jsonMod "encoding/json"
+	"fmt"
 	"io/ioutil"
+	"path/filepath"
 	"regexp"
 	"strings"
 	"testing"
@@ -39,6 +42,10 @@ func TestCaddyfileAdaptToJSON(t *testing.T) {
 		// replace windows newlines in the json with unix newlines
 		json = winNewlines.ReplaceAllString(json, "\n")
 
+		// replace os-specific default path for file_server's hide field
+		replacePath, _ := jsonMod.Marshal(fmt.Sprint(".", string(filepath.Separator), "Caddyfile"))
+		json = strings.ReplaceAll(json, `"./Caddyfile"`, string(replacePath))
+
 		// run the test
 		ok := caddytest.CompareAdapt(t, filename, caddyfile, "caddyfile", json)
 		if !ok {