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

modules/caddyhttp/encode/gzip/gzip_precompressed.go

@@ -0,0 +1,28 @@
+package caddygzip
+
+import (
+	"github.com/caddyserver/caddy/v2"
+	"github.com/caddyserver/caddy/v2/modules/caddyhttp/encode"
+)
+
+func init() {
+	caddy.RegisterModule(GzipPrecompressed{})
+}
+
+// GzipPrecompressed provides the file extension for files precompressed with gzip encoding.
+type GzipPrecompressed struct {
+	Gzip
+}
+
+// CaddyModule returns the Caddy module information.
+func (GzipPrecompressed) CaddyModule() caddy.ModuleInfo {
+	return caddy.ModuleInfo{
+		ID:  "http.precompressed.gzip",
+		New: func() caddy.Module { return new(GzipPrecompressed) },
+	}
+}
+
+// Suffix returns the filename suffix of precompressed files.
+func (GzipPrecompressed) Suffix() string { return ".gz" }
+
+var _ encode.Precompressed = (*GzipPrecompressed)(nil)