AsyncValidation

[1eyewonder]

Proposed Changes

Added AsyncValidation functionality per request of #209

Types of changes

What types of changes does your code introduce to FsToolkit.ErrorHandling?
Put an x in the boxes that apply and remove ones that don't apply

• Bugfix (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)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• Build and tests pass locally
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have added necessary documentation (if appropriate)

Further comments

Majority of code was copied from Validation functions and converted to async. I'm sure there are plenty of things I'll need to address on the PR, but I wanted to get the code out there to start the conversation/reviews. I'm more than happy helping on documentation as well, I just figured I'd get a thumbs up on implementation first from the maintainers.

[TheAngryByrd]

Thanks for this! Left some feedback on changes needed for this.

[1eyewonder]

Done! Would the next steps be adding documentation in the gitbook folder?

[TheAngryByrd]

Yeah some documentation for that would be nice too :)