…yml`

This brings the Export config object more in line with how other config
objects are specified in the unparsed definition. It allows for specifying
of extra configs, although they won't get propagate to the final config.

This allows for specifying `ExportConfig` options at the `SavedQueryConfig` level.
This also therefore allows these options to be specified in the dbt_project.yml
config. The plan in the follow up commit is to merge the `SavedQueryConfig` options
into all configs of `Exports` belonging to the saved query.

There are a couple caveots to call out:
1. We've used `schema` instead of `schema_name` on the `SavedQueryConfig` despite
it being called `schema_name` on the `ExportConfig`. This is because need `schema_name`
to be the name of the property on the `ExportConfig`, but `schema` is the user facing
specification.
2. We didn't add the `ExportConfig` `alias` property to the `SavedQueryConfig` This
is because `alias` will always be specific to a single export, and thus it doesn't
make sense to allow defining it on the `SavedQueryConfig` to then apply to all
`Exports` belonging to the `SavedQuery`

…om project config

Export configs will now inherit from saved query configs, with a preference
for export config specifications. That is to say an export config will inherity
a config attr from the saved query config only if a value hasn't been supplied
on the export config directly. Additionally because the saved query config has
a similar relationship with the project config, exports configs can inherit
from the project config (again with a preference for export config specifications).

…a_name

I somehow wrote a really weird, but also valid, conditional statement. Previously
the conditional was
```
if combined.get("schema") is not combined.get("schema_name") is None:
```
which basically checked whether `schema` was a boolean that didn't match
the boolean of whether `schema_name` was None. This would pretty much
always evaluate to True because `schema` should be a string or none, not
a bool, and thus would never match the right hand side. Crazy. It has now
been fixed to do the thing we want to it to do. If `schema` isn't `None`,
and `schema_name` is `None`, then set `schema_name` to have the value of
`schema`.