crash when parsing custom validators

There is a bug in `ckanext.spatial.harvesters.base.SpatialHarvester._get_validator()`

https://github.com/ckan/ckanext-spatial/blob/09fbc27054f5d2f41238d858b018fca94a7c4c0d/ckanext/spatial/harvesters/base.py#L790

This is a verbatim copy of the code that is currently in master:

```python
# Add any custom validators from extensions
for plugin_with_validators in p.PluginImplementations(ISpatialHarvester):
custom_validators = plugin_with_validators.get_validators()
for custom_validator in custom_validators:
if custom_validator not in all_validators:
self._validator.add_validator(custom_validator)
```

In the above snippet, the `custom_validators` variable is a Python dictionary, [as documented in the CKAN docs](https://docs.ckan.org/en/2.9/extensions/plugin-interfaces.html#ckan.plugins.interfaces.IValidators.get_validators). The problem is that the nested `for` loop is iterating over the dictionary keys, which are strings, and then passes each item to `self._validator.add_validator`, which expects to receive a class instance instead - which by the way is also bugged, as current versions of CKAN allow using regular functions as validators.

I'm guessing this is something left behind from when CKAN moved to Flask from Pylons?

Unfortunately this is something that prevents using custom validators and customizing a harvester behavior at the same time in the same plugin class.

Start in ckanext/spatial/harvesters/base.py at SpatialHarvester._get_validator(), especially the custom-validator loop around line 790. Reproduce the crash with a plugin that provides validators while also customizing harvester behavior, then check the existing CKAN validator interface expectations. Done means custom validators are accepted without the parsing crash and the relevant validation behavior remains functional.

Written by the indexing model from the issue text.