[RFC] Allow validators to operate on list items - was: `AttributeError: 'BoxList' object has no attribute 'get'`on calling validate()

settings.toml
```toml
[[PROVIDERS]]
PROVIDER_NAME = "A"
TYPE = "1"

[[PROVIDERS]]
PROVIDER_NAME = "B"
TYPE = 1
```

app.py
```python
from dynaconf import Dynaconf, Validator
settings = Dynaconf()

settings.validators.register(Validator('PROVIDERS.TYPE', is_type_of=str))
settings.validators.validate()
```

throws `AttributeError: 'BoxList' object has no attribute 'get'`

Expected behaviour would be, that dynaconf runs the defined validator on all values inside the list of dicts of PROVIDERS.