Add BsonArray.Add(string, BsonValue) overload

For simpler BsonArray initialization by taking advantage of C# collection initializer.

So we can simplify BsonArray initializing from this :

new BsonArray

{

    new BsonDocument("a", 1),

    new BsonDocument("b", 2),

};

To this :

new BsonArray

{

    { "a", 1 },

    { "b", 2 },

};