[epub_view] Add support for opening books from URL

[royacodes]

This pull request adds a new feature to the epub_view library, allowing users to open books directly from a URL.

Changes Made

• Added a new option to load books from a URL.
• Implemented URL parsing and book loading logic.
• Included usage examples in the documentation.

Usage Example

import 'package:epub_view/epub_view.dart';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart' show SystemChrome, SystemUiOverlayStyle;

void main() => runApp(const MyApp());

class MyApp extends StatefulWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> with WidgetsBindingObserver {
  @override
  void initState() {
    WidgetsBinding.instance.addObserver(this);
    super.initState();
  }

  @override
  void dispose() {
    WidgetsBinding.instance.removeObserver(this);
    super.dispose();
  }

  @override
  void didChangePlatformBrightness() {
    _setSystemUIOverlayStyle();
  }

  Brightness get platformBrightness =>
      MediaQueryData.fromView(WidgetsBinding.instance.window)
          .platformBrightness;

  void _setSystemUIOverlayStyle() {
    if (platformBrightness == Brightness.light) {
      SystemChrome.setSystemUIOverlayStyle(SystemUiOverlayStyle(
        statusBarBrightness: Brightness.light,
        statusBarIconBrightness: Brightness.dark,
        systemNavigationBarColor: Colors.grey[50],
        systemNavigationBarIconBrightness: Brightness.dark,
      ));
    } else {
      SystemChrome.setSystemUIOverlayStyle(SystemUiOverlayStyle(
        statusBarBrightness: Brightness.dark,
        statusBarIconBrightness: Brightness.light,
        systemNavigationBarColor: Colors.grey[850],
        systemNavigationBarIconBrightness: Brightness.light,
      ));
    }
  }

  @override
  Widget build(BuildContext context) => MaterialApp(
        title: 'Epub demo',
        theme: ThemeData(
          primarySwatch: Colors.blue,
          brightness: Brightness.light,
        ),
        darkTheme: ThemeData(
          primarySwatch: Colors.blue,
          brightness: Brightness.dark,
        ),
        debugShowCheckedModeBanner: false,
        home: const MyHomePage(),
      );
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key}) : super(key: key);

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  late EpubController _epubReaderController;

  @override
  void initState() {
    _epubReaderController = EpubController(
      document:
           EpubDocument.openUrl('https://www.gutenberg.org/ebooks/11.epub.images'),
    );
    super.initState();
  }

  @override
  void dispose() {
    _epubReaderController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) => Scaffold(
        appBar: AppBar(
          title: EpubViewActualChapter(
            controller: _epubReaderController,
            builder: (chapterValue) => Text(
              chapterValue?.chapter?.Title?.replaceAll('\n', '').trim() ?? '',
              textAlign: TextAlign.start,
            ),
          ),
          actions: <Widget>[
            IconButton(
              icon: const Icon(Icons.save_alt),
              color: Colors.white,
              onPressed: () => _showCurrentEpubCfi(context),
            ),
          ],
        ),
        drawer: Drawer(
          child: EpubViewTableOfContents(controller: _epubReaderController),
        ),
        body: EpubView(
          builders: EpubViewBuilders<DefaultBuilderOptions>(
            options: const DefaultBuilderOptions(),
            chapterDividerBuilder: (_) => const Divider(),
          ),
          controller: _epubReaderController,
        ),
      );

  void _showCurrentEpubCfi(context) {
    final cfi = _epubReaderController.generateEpubCfi();

    if (cfi != null) {
      ScaffoldMessenger.of(context).showSnackBar(
        SnackBar(
          content: Text(cfi),
          action: SnackBarAction(
            label: 'GO',
            onPressed: () {
              _epubReaderController.gotoEpubCfi(cfi);
            },
          ),
        ),
      );
    }
  }
}