Split "modules/context.go" to separate files (#24569)

The "modules/context.go" is too large to maintain.

This PR splits it to separate files, eg: context_request.go,
context_response.go, context_serve.go

This PR will help:

1. The future refactoring for Gitea's web context (eg: simplify the context)
2. Introduce proper "range request" support
3. Introduce context function

This PR only moves code, doesn't change any logic.

modules/context/context_serve.go

@@ -0,0 +1,74 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package context
+
+import (
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"strconv"
+	"strings"
+	"time"
+
+	"code.gitea.io/gitea/modules/httpcache"
+	"code.gitea.io/gitea/modules/typesniffer"
+)
+
+type ServeHeaderOptions struct {
+	ContentType        string // defaults to "application/octet-stream"
+	ContentTypeCharset string
+	ContentLength      *int64
+	Disposition        string // defaults to "attachment"
+	Filename           string
+	CacheDuration      time.Duration // defaults to 5 minutes
+	LastModified       time.Time
+}
+
+// SetServeHeaders sets necessary content serve headers
+func (ctx *Context) SetServeHeaders(opts *ServeHeaderOptions) {
+	header := ctx.Resp.Header()
+
+	contentType := typesniffer.ApplicationOctetStream
+	if opts.ContentType != "" {
+		if opts.ContentTypeCharset != "" {
+			contentType = opts.ContentType + "; charset=" + strings.ToLower(opts.ContentTypeCharset)
+		} else {
+			contentType = opts.ContentType
+		}
+	}
+	header.Set("Content-Type", contentType)
+	header.Set("X-Content-Type-Options", "nosniff")
+
+	if opts.ContentLength != nil {
+		header.Set("Content-Length", strconv.FormatInt(*opts.ContentLength, 10))
+	}
+
+	if opts.Filename != "" {
+		disposition := opts.Disposition
+		if disposition == "" {
+			disposition = "attachment"
+		}
+
+		backslashEscapedName := strings.ReplaceAll(strings.ReplaceAll(opts.Filename, `\`, `\\`), `"`, `\"`) // \ -> \\, " -> \"
+		header.Set("Content-Disposition", fmt.Sprintf(`%s; filename="%s"; filename*=UTF-8''%s`, disposition, backslashEscapedName, url.PathEscape(opts.Filename)))
+		header.Set("Access-Control-Expose-Headers", "Content-Disposition")
+	}
+
+	duration := opts.CacheDuration
+	if duration == 0 {
+		duration = 5 * time.Minute
+	}
+	httpcache.SetCacheControlInHeader(header, duration)
+
+	if !opts.LastModified.IsZero() {
+		header.Set("Last-Modified", opts.LastModified.UTC().Format(http.TimeFormat))
+	}
+}
+
+// ServeContent serves content to http request
+func (ctx *Context) ServeContent(r io.ReadSeeker, opts *ServeHeaderOptions) {
+	ctx.SetServeHeaders(opts)
+	http.ServeContent(ctx.Resp, ctx.Req, opts.Filename, opts.LastModified, r)
+}