Add support for validation service

[richardhuaaa]

This just implements the wrappers for the validation service, and plumbs it into the identity service. I will be out for the next two weeks, feel free to modify or land on my behalf.

I wasn't able to complete the remaining piece of the identity service, this is what needs to be done in the next PR:

1. Implement the TODO's here and here, using the logic described here. The validation can be done by calling into the handler introduced in this PR.
2. Implement this read

[neekolas]

This looks safe to build on top of. Think we can merge and keep going to actually integrate the validation service into PublishIdentityUpdate