Allow Custom Token Length in Sanctum Token Generation

[Usmanzahidcode]

This update allows the token length in generateTokenString to be configurable via the sanctum.entropy_length config variable. It defaults to 40 for backward compatibility while enabling developers to define custom lengths for enhanced security.

Custom lengths provide users the flexibility to enhance token security or align with their application's unique requirements, such as stricter compliance standards or integration needs.

The change is fully backwards-compatible and simplifies token customization for varied application needs. All tests passed.

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If applicable, please consider releasing your code as a package so that the community can still take advantage of your contributions!

[Usmanzahidcode]

Thank you. I will be putting this up as a separate package.