Endpoint to validate json

[helen-m-lin]

Is your feature request related to a problem? Please describe.
As a user, I would like to validate my job request before I submit it.

Describe the solution you'd like
Endpoint to validate raw json against aind-data-transfer-models. It should return the validated json or errors if request is invalid.

Describe alternatives you've considered
Leaving it as is.

Additional context
Originally part of #107 but rescoped to just the validate endpoint.