Add support for logging

[nielsenko]

You can set logger to use with Realm like this:

final logger = Logger('x');
logger.root.level = RealmLogLevel.trace;
Realm.logger = logger;

If nothing is specified, the default logger traces info messages or worse to the console.

[nirinchev]

This looks okay, but I have some suggestions for making the API nicer. Also, can we add a test for logging to a file as well as a test where the sync log level differs from the logger log level.

[desistefanova]

Just a reminder to change default logLevel from error to info.

/// Instantiates a new [AppConfiguration] with the specified appId.
  AppConfiguration(
    this.appId, {
    Uri? baseUrl,
    Directory? baseFilePath,
    this.defaultRequestTimeout = const Duration(seconds: 60),
    this.localAppName,
    this.localAppVersion,
    this.metadataEncryptionKey,
    this.metadataPersistenceMode = MetadataPersistenceMode.plaintext,
    this.logLevel = LogLevel.info,
    this.maxConnectionTimeout = const Duration(minutes: 2),
    HttpClient? httpClient,
  })  : baseUrl = baseUrl ?? Uri.parse('https://realm.mongodb.com'),
        baseFilePath = baseFilePath ?? Directory(Configuration.filesPath),
        httpClient = httpClient ?? HttpClient();

[nirinchev]

This doesn't appear to be correct anymore now that we have a single logger.