Fix setting warn_error_options via dbt_project.yaml flags.

Back when I did the work for #10058 (specifically c52d653) I thought that
the `warn_error_options` would automatically be converted from the yaml
to the `WarnErrorOptions` object as we were building the `ProjectFlags` object,
which holds `warn_error_options`, via `ProjectFlags.from_dict`. And I thought
this was validated by the `test_can_silence` test added in c52d653. However,
there were two problems:

1. The definition of `warn_error_options` on `PrjectFlags` is a dict, not a
`WarnErrorOptions` object
2. The `test_can_silence` test was broken, and not testing what I thought

The quick fix (this commit) is to ensure `silence` is passed to `WarnErrorOptions`
instantiation in `dbt.cli.flags.convert_config`. The better fix would be to make
the `warn_error_options` of `ProjectFlags` a `WarnErrorOptions` object instead of
a dict. However, to do this we first need to update dbt-common's `WarnErrorOptions`
definition to default `include` to an empty list. Doing so would allow us to get rid
of `convert_config` entirely.

core/dbt/cli/flags.py

@@ -57,6 +57,7 @@ def convert_config(config_name, config_value):
         ret = WarnErrorOptions(
             include=config_value.get("include", []),
             exclude=config_value.get("exclude", []),
+            silence=config_value.get("silence", []),
             valid_error_names=ALL_EVENT_NAMES,
         )
     return ret