[http-client-csharp] Strong typed snippet methods

[ArcturusZhang]

We have some methods with semantic meanings to construct a statement, such as

public static MethodBodyStatement Declare(string name, TypedValueExpression value, out TypedValueExpression variable)

this loses its type information, for instance, when I put a StreamExpression as value, we only get a TypedValueExpression as the variable.

If possible, we could introduce a generic version of this method, such as

public static MethodBodyStatement Declare<T>(string name, T value, out T variable) where T : TypedValueExpression

therefore we could get this usage:

Declare("stream", streamExpression, out StreamExpression streamVar);