SDK6: Have Parameters implement IEnumerable to allow for collection initializers

[almostchristian]

Our testing code and application code use collection initializers with Parameters resource which was possible in FHIR SDK 5.x. In SDK 6, this is no longer possible since Base no longer implements IDictionary<string,object>, and in turn IEnumerable which is required for collection initializers.

// SDK 5
var parameters = new Parameters
{
   { "value1", new FhirString("test") },
   { "value2", new Integer(1) }, 
   { "value3", [ Tuple.Create<string, Base>("value3.1", new FhirString("a")) ] }
};

// SDK 6
var parameters = new Parameters();
parameters.Add("value1", new FhirString("test"));
parameters.Add("value2", new Integer(1));
parameters.Add("value3", [ Tuple.Create<string, Base>("value3.1", new FhirString("a")) ]);

Describe alternatives you've considered
In our testing code, we were able to mitigate the issue by adding a new class Parameters2 which approximates the previous functionality with minimal code changes. Production code however would not be able to use this class and would have to go back to the old way of initializing the parameter contents.

internal class Parameters2 : IEnumerable
{
    public Parameters Parameters { get; } = new Parameters();

    public void Add(string name, Base value)
    {
        Parameters.Add(name, value);
    }

    public void Add(string name, IEnumerable<Tuple<string, Base>> value)
    {
        Parameters.Add(name, value);
    }

    IEnumerator IEnumerable.GetEnumerator()
        => Parameters.EnumerateElements().GetEnumerator();

    public static implicit operator Parameters(Parameters2 parameters2)
        => parameters2.Parameters;
}