refactor(httpheader): extract header parsing into dedicated package

Moved HTTP header parsing and building logic from server package to internal/httpheader

internal/httpheader/request.go

@@ -0,0 +1,49 @@
+package httpheader
+
+import (
+	"bufio"
+	"fmt"
+)
+
+func NewRequestHeader(r interface{}) (RequestHeader, error) {
+	switch v := r.(type) {
+	case []byte:
+		return parseHeadersFromBytes(v)
+	case *bufio.Reader:
+		return parseHeadersFromReader(v)
+	default:
+		return nil, fmt.Errorf("unsupported type: %T", r)
+	}
+}
+
+func (req *requestHeader) Value(key string) string {
+	val, ok := req.headers[key]
+	if !ok {
+		return ""
+	}
+	return val
+}
+
+func (req *requestHeader) Set(key string, value string) {
+	req.headers[key] = value
+}
+
+func (req *requestHeader) Remove(key string) {
+	delete(req.headers, key)
+}
+
+func (req *requestHeader) GetMethod() string {
+	return req.method
+}
+
+func (req *requestHeader) GetPath() string {
+	return req.path
+}
+
+func (req *requestHeader) GetVersion() string {
+	return req.version
+}
+
+func (req *requestHeader) Finalize() []byte {
+	return finalize(req.startLine, req.headers)
+}