Add isDefaultSite filter to sites query

[ianmeigh]

Adds an optional isDefaultSite: Boolean argument to the sites GraphQL query, allowing sites to be filtered by their default status.

This is useful for multi-site Wagtail setups where we want to query only the default site for things like global nav, settings, or fallback content, without knowing its hostname or id.

[jams2]

Thanks @ianmeigh . Could you please update the changelog and the documentation - we should document the query arg as shown in the site query example.