Support configurable locales per Source/Stream

## What?
Provide functionality that allows users to configure the locale for a given Source or Stream. This locale should then be usable in processing pipelines in order to avoid errors while parsing dates.

## Why?
Graylog sometimes runs in mixed-locale environments where logs from one source are localized in one language while logs from another source are localized in a different language. While the `parse_date` pipeline function supports a locale parameter, there is no way to construct a single pipeline that calls `parse_date` and functions correctly when applied to Streams with differing locales. The problem becomes especially pronounced when considering Illuminate processing, where customers do not even have the option to update the pipeline and provide a locale parameter to the `parse_date` function.