Description of Linked Ticket

Currently, 'near' has to be a literal array (or GeoJSON object):

> db.c.aggregate([ {$geoNear: {near: [0, 0]}} ]

Instead we should allow it to be an expression:

> db.c.aggregate([ {$geoNear: {near: "$$pt"}} ], {let: {pt: [0, 0]}})

It doesn't make sense to allow the query point to vary per-document, so it has to be a constant expression.

We also want to allow 'near' to be bound in a $lookup:

{$lookup: {
  from: 'coll',
  as: 'docs',
  let: {pt: "$location"},
  pipeline: [
    {$geoNear: {
      near: "$$pt",
      ...
    }}
  ]
}}

The expression "$$pt" is constant within each subquery.

To make this work I think we'll want to:

• Change the DocumentSourceGeoNear parser to accept any expression, for 'near'.
• Change DocumentSourceGeoNear::optimize to optimize the expression.
• When we convert DocumentSourceGeoNear to DocumentSourceGeoNearCursor, raise an error if the expression is not a constant.