feat(landmark_manager): add get_landmarrks func

[YamatoAndo]

Description

A function has been added to return an array of landmarks in the landmark_manager.
This function is intended to be used in the lidar_marker_localizer for searching the nearest landmark and so on.

Tests performed

LSim works.

Effects on system behavior

Not applicable.

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[SakodaShintaro]

Looks Good To Me

[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (b51759f) 14.65% compared to head (f09952d) 14.64%.

Files	Patch %	Lines
...ocalizer/landmark_manager/src/landmark_manager.cpp	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6063      +/-   ##
==========================================
- Coverage   14.65%   14.64%   -0.01%     
==========================================
  Files        1857     1858       +1     
  Lines      126512   126520       +8     
  Branches    37059    37063       +4     
==========================================
  Hits        18535    18535              
- Misses      87154    87162       +8     
  Partials    20823    20823              

Flag	Coverage Δ		*Carryforward flag
differential	5.61% <0.00%> (?)
total	14.65% <ø> (ø)		Carriedforward from b51759f

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