Allow declaring parameters with uninitialized values

[jacobperron]

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.

[jacobperron]

Follow-up from #1588

[jacobperron]

Although this would be a behavior change, IMO, we should backport this to Galactic. My reasoning is that, currently, there is no way to support "optional" parameters besides allowing undeclared parameters or optionally declaring parameters (I consider the latter as an anti-pattern as discussed in #1588). Therefore, without this feature users will have to resort to less than desirable solutions for scenarios where they need optional parameter values. Prior to Galactic, this feature was possible as users could declare a parameter with an unset type with:

node->declare_parameter("my_param");

[jacobperron]

For reference, this is the original PR adding static typing of parameters: #1522