Fix bson.D conversion in GRPC

[VinozzZ]

What does this change

MongoDB expects to receive bson.D for $sort and $group in a aggregation pipeline.
Prior to this PR, after the grpc server receives data from a client, it does not convert the data from []interface{} back to bson.D, which caused the query to fail.
This PR fixes this issue and adds tests to check for proper conversion.

What issue does it fix

Closes #2063

Notes for the reviewer

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• Did you write documentation?
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Reviewer Checklist

• Comment with /azp run test-porter-release if a magefile or build script was modified
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[carolynvs]

The CI build is failing but this looks like it's going in the right direction

[carolynvs]

I don't think that it's possible for c to be nil here? If you call the function without passing any optional arguments the value is an empty array, not nil.

I noticed that in AsMap you always use the ConvertFloatToInt converter, even when they specify the optional custom converter parameter. Since you are passing c through to AsMap, I believe that this results in ConvertFloatToInt being used twice in AsMap when c is nil (which is never as I said above).

I'm pretty sure that you can delete these lines here and it will work exactly the same.

Suggested change

	if c == nil {
	c = []converter{ConvertFloatToInt}
	}

[carolynvs]

Just one thing to clear up and then this is ready to merge! 🚀

[carolynvs]

🤞