Set initial pose from params

[ivanpauno]

Fixes #41.
I also want to have this available for #35 (comment).

Summary

Allow to set the initial pose from ROS parameters.
The covariance parameters does not seem to be supported by nav2_amcl, but I think it's a good idea to have them.
The default values of the covariance parameters were taken from rviz defaults https://github.com/ros2/rviz/blob/a9a4d9e8ff09de9b134d022a3ec856ee0c3d3729/rviz_default_plugins/src/rviz_default_plugins/tools/pose_estimate/initial_pose_tool.cpp#L62-L70, which seem pretty reasonable.

Example: ros2 launch beluga_example example_rosbag_launch.py amcl.initial_pose.y:=2. amcl.initial_pose.yaw:=3.14 amcl.set_initial_pose:=true

Checklist

• Read the contributing guidelines.
• Configured pre-commit and ran colcon test locally.
• Signed all commits for DCO.
• Added tests (regression tests for bugs, coverage of new code for features).
• Updated documentation (as needed).
• Checked that CI is passing.

[nahueespinosa]

@ivanpauno Left some comments!

[nahueespinosa]

LGTM!