Rewrite credentials management

[ghostzero]

This is an Intermediate feature (opt-in). Most user will not use it, unless you use Laravel-Twitch on large environments, like i do.

Hey, I have the use-case where i need to override the credentials management based on different providers, like "Google Secret Manager" and other providers like AWS, or even custom providers. In fact I have different projects that use different stategies, and with this PR I want to add basic support into the core of this lib.

Currently it's not possible to request a new twitch app access token if someone use the non-twitch provider. Most apps, where i implemented a custom secret manager, or uses google/aws has already their own key rotation system. So there was no need to implement that right now.

This PR contains:

• Rewrite credentials management
• Drop php 7 support, since it's EOL
• Add amazon secret manager support
• Add google secret manager support
• Add documentaion on how to use the secret managers