[ControllerInterface] Move critical variables to the private context

[saikishor]

I think we should move these critical variables to the private context to avoid the overidding classes to be able to modify them. Some of these parameters might affect the CM

We could already access the data of these variables through the provided methods

ros2_control/controller_interface/src/controller_interface_base.cpp

Lines 133 to 137 in 9d1aff9

	unsigned int ControllerInterfaceBase::get_update_rate() const { return update_rate_; }
	bool ControllerInterfaceBase::is_async() const { return is_async_; }
	const std::string & ControllerInterfaceBase::get_robot_description() const { return urdf_; }

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.56%. Comparing base (1f8ca08) to head (7e37577).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1623      +/-   ##
==========================================
+ Coverage   87.30%   87.56%   +0.25%     
==========================================
  Files         108      108              
  Lines        9866     9924      +58     
  Branches      890      891       +1     
==========================================
+ Hits         8614     8690      +76     
+ Misses        929      917      -12     
+ Partials      323      317       -6     

Flag	Coverage Δ
unittests	87.56% <ø> (+0.25%)	⬆️

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Files	Coverage Δ
...controller_interface/controller_interface_base.hpp	87.50% <ø> (ø)

... and 2 files with indirect coverage changes

[bmagyar]

please make sure to update controllers too if needed

[saikishor]

please make sure to update controllers too if needed

@bmagyar I've already done it. It's in this PR: ros-controls/ros2_controllers#1221