[Draft]Add structured parameter support

[Rahul-K-A]

Description

Adds support for YAML structures as a parameter type. Original GSOC proposal here. Essentially with a YAML file such as

<node>:
  ros__parameters:
    passenger_size:
      min:
        x: 0.6
        y: 0.4
        z: 0.17
      max:
        x: 0.850
        y: 0.65
        z: 0.47

The user will now able to access min and max as individual parameters. These parameters will be available in rclcpp as a YAML::Node or as a dict() in Python. The rcl specific changes primarily include modifications to rcl parser that:

1. Add support for strucrured yaml field in rcl_variant_t as a C string (char *)
2. Modifies the YAML document parsing code to detect nesting
3. If a nesting of parameters it detected, the parser will automatically write all subsequent events to the structured yaml events
4. The parser uses the indentations (map_depth) to determine when to stop writing to the YAML structure

Support for namespaced existing namespaced parameters is not affected. If required users will still be able to access individual parameters such as passenger_size.min.x, passenger_size.max.x, etc.

Fixes # (issue)

Is this user-facing behavior change?

Somewhat(?). The new changes will allow the user to define use new parameter types. Old methods of getting parameters still work

Did you use Generative AI?

No.

Additional Information

1. Companion rclcpp PR: [Draft]Add structured parameter support rclcpp#2944
2. Companion rclpy PR: [Draft]Add structured parameter support rclpy#1494
3. Compantion rcl_interfaces PR: Add structured parameter support  rcl_interfaces#183

[Rahul-K-A]

Tagging this as a draft, because Im yet to add in tests/ apply linter. Once done,will remove the draft tag. Maintainers are welcome to start reviewing it in the meantime

[asherikov]

1. does this work?

<node>:
  ros__parameters:
    passenger_size:
       - x: 0.6
       - x: 0.850

2. "The parser uses the indentations": do you mean nesting? yaml may be in compact form or JSON
3. "automatically write all subsequent events to the structured yaml events": not clear, when the folowing is double and when it is yaml?

<node>:
  ros__parameters:
    passenger_size:
      min:
        x: 0.6

[jmachowinski]

Rough first pass, need to look at the code in more detail

[jmachowinski]

Suggested change

	char * yaml_value; ///< If structured YAML, gets stored here -> Is this right?
	char * yaml_value; ///< If structured YAML, gets stored here

[jmachowinski]

is this todo solved or still relevant ?

[jmachowinski]

Suggested change

	// This is done for optimization (?) - Rahul-K-A
	// This is done for optimization

[jmachowinski]

Suggested change

	size_t max_string_length = 100000;
	const size_t max_string_length = 100000;

[jmachowinski]

Suggested change

	unsigned char nested_param_string_allocator[100000];
	unsigned char nested_param_string_allocator[max_string_length];

Or is this a cpp feature and does not work for C code ?

[jmachowinski]

Suggested change

	size_t max_string_length = 100000;
	const size_t max_string_length = 100000;

[jmachowinski]

Suggested change

	char nested_param_string_allocator[100000];
	char nested_param_string_allocator[max_string_length];

might not work in C