Add default argument plugins

Problem/Motivation

Rather than hardcoding a number of place in a URL for webform/submission ID default value in a contextual filter, it'd be better to have a plugin, in case a route changes.

Steps to reproduce

Add a contextual filter for webform ID or webform submission ID. To set the default value from URL, you currently have to select "Raw value from URL" and choose a number for "path component".

Proposed resolution

See MR.