Add COALESCE statement support to Linq

[KenshinRus]

Looks like there is nothing available in Marten linq extensions to support coalescing operator ?? when select fields, aka

COALESCE(d.data -> 'LastApprovedRequest' ->> 'Status', d.data ->> 'Status')

[jeremydmiller]

Correct. That's never come up before. Think this is an "I take pull requests" moment.