feat: allow async factory function on forRoot

[stelescuraul]

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.
Allow the passed factory function to be async.

Type of change

Please select the options that are relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (non-breaking change which does not add functionality)

Checklist:

• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works