Add support for relative date filter definitions in Meltano Filters

Merge Request Checklist

• Link to an issue (#1830 (closed))
• Include the proposed fix or feature
• Include and update tests for the modified code
• Include a documentation change
• Add a CHANGELOG.md entry in the Unreleased section

This MR adds support for relative date filter definitions in Meltano Filters.

That means that filters over dates and times can have a [+-]N[dmy] format instead of a fixed date.

That allows Meltano to generate a date relative to a pivot date provided by in the query definition or NOW().