selfservice: Form BodyParser should assert types using JSON Schema

[aeneasr]

Describe the bug

At the moment, selfservice.BodyParser assert types based on possible conversion. "true" is always converted to true, "12345" always to 12345. This doesn't make sense because users might use a random number as their e.g. name (due to laziness or privacy reasons). Instead of asserting based on user input, we need to assert types based on the JSON Schema for that identity.

Reproducing the bug

Steps to reproduce the behavior:

1. Create a JSON Schema that accepts a string as a trait
2. Send form data containing only numbers for that trait

Expected behavior

The types should be set correctly and the JSON Schema should be used as a source of truth, not the user input.

Additional context

Follow up to #98

If you wish to tackle this please comment here.