Add back resource_exists validator for uri field on File

[evamaxfield]

We had to remove the usage of resources_exists from the uri field validator because we couldn't think of a method to pass the google credentials to the validator. At some point we may want to add this back but this shouldn't hold up #49 from being merged.

Actually after trying this, I think passing another param to the validator is a little tricky.

I can add kwargs to resource_exists, but I'm not sure if there's an easy way to pass in fs into that function inside uri = fields.TextField(required=True, validator=validators.resource_exists).

Because model fields in FireO are assigned like db_file.uri = "gs:https://stuff", I'm not sure we can pass in other params into the validators arg.

Would it make more sense to just validate separately outside of FireO's built-in field validation? Or is there an easier way to do this I'm missing?

Originally posted by @isaacna in #49 (comment)

[isaacna]

Based on FireO's implementation, I don't think it's possible to pass in kwargs to a validator method passed in.

Two things we could do are:

1. Make a PR to FireO to support extra validator args
2. Handle validation on database models on our own. We could do something like create a base class with a validator field that takes in a callable, and a validator_args field. Then we can have each database model we have also inherit from that class.

I think 2 would be the easier option and give us more options to customize to our use cases in the future.

[isaacna]

I added an option to pass kwargs to validator functions in this PR, so this should be doable now.