feat(route_handler): add preferred lane getter function

[satoshi-ota]

Description

add preferred lane getter function.

lanelet::ConstLanelets RouteHandler::getPreferredLanelets() const
{
  return preferred_lanelets_;
}

Tests performed

Nothing.

Effects on system behavior

Nothing.

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 14.64%. Comparing base (fdd3f9e) to head (dc8b081).
Report is 4 commits behind head on main.

Files	Patch %	Lines
planning/route_handler/src/route_handler.cpp	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6496      +/-   ##
==========================================
- Coverage   14.64%   14.64%   -0.01%     
==========================================
  Files        1899     1899              
  Lines      130283   130285       +2     
  Branches    38311    38311              
==========================================
  Hits        19076    19076              
- Misses      89793    89795       +2     
  Partials    21414    21414              

Flag	Coverage Δ		*Carryforward flag
differential	15.21% <0.00%> (?)
total	14.64% <ø> (ø)		Carriedforward from fdd3f9e

*This pull request uses carry forward flags. Click here to find out more.

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