Feature request

Feature description

There are situations where we want to declare a parameter, but not initialize it until later.

For example, it would be nice to declare a parameter, with a type like this without it throwing an exception because the user did not provide an override at runtime:

node->declare_parameter(name, type);

We could then later set the value of the parameter programmatically from the node itself (e.g. with set_parameter) or let the user set it later with a service call (e.g. ros2 param set).

Implementation considerations

After declaring a parameter without an initial value and if there is no user-provided override, then I would expect the parameter to have a value of "UNSET", and trying to access a value might throw an exception.