feat: add log handler to be able to use a custom logger

[nicobritos]

• Add custom logger handler to use a custom Logger instead of printing to console.
• Hide PusherChannelsPackageLogger from public API.
• Fix logger tests not capturing what was being printed to console.
• Remove logTest method as it is not needed anymore.

[kerimamansaryyev]

Changed just an export to expose all structures from configs.dart for api library. Because it is used for development

[kerimamansaryyev]

@nicobritos Thank you very much! I will mention this contribution in CHANGELOG.md