refactor(radar_object_clustering)!: fix namespace and directory structure

[technolojin]

Description

This PR puts headers in the autoware namespace.
Part of: autowarefoundation/autoware#4569

Also, align directory structure to follow the coding guidelines.

Tests performed

Not applicable.

Effects on system behavior

Not applicable.

Interface changes

Not applicable.

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 0.00%. Comparing base (507e3f4) to head (4fe6beb).
Report is 145 commits behind head on main.

Files	Patch %	Lines
...ct_clustering/src/radar_object_clustering_node.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #7654       +/-   ##
==========================================
- Coverage   14.84%   0.00%   -14.85%     
==========================================
  Files        1999      57     -1942     
  Lines      139163    3247   -135916     
  Branches    43716     219    -43497     
==========================================
- Hits        20661       0    -20661     
+ Misses      95731    3247    -92484     
+ Partials    22771       0    -22771     

Flag	Coverage Δ
differential	0.00% <0.00%> (?)
total	?

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[scepter914]

LGTM
(Note: without checking acting, after integrating with other PRs and checking acting this PR can be merged)