refactor(object_velocity_splitter)!: 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.

Pre-review checklist for the PR author

The PR author must check the checkboxes below when creating the PR.

• I've confirmed the contribution guidelines.
• The PR follows the pull request guidelines.

In-review checklist for the PR reviewers

The PR reviewers must check the checkboxes below before approval.

• The PR follows the pull request guidelines.

Post-review checklist for the PR author

The PR author must check the checkboxes below before merging.

• There are no open discussions or they are tracked via tickets.

After all checkboxes are checked, anyone who has write access can merge the PR.

[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 (0335b07).
Report is 145 commits behind head on main.

Files	Patch %	Lines
...ity_splitter/src/object_velocity_splitter_node.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #7646       +/-   ##
==========================================
- Coverage   14.84%   0.00%   -14.85%     
==========================================
  Files        1999      32     -1967     
  Lines      139163    1773   -137390     
  Branches    43716      68    -43648     
==========================================
- Hits        20661       0    -20661     
+ Misses      95731    1773    -93958     
+ Partials    22771       0    -22771     

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[github-actions]

Thank you for contributing to the Autoware project!

🚧 If your pull request is in progress, switch it to draft mode.

Please ensure:

• You've checked our contribution guidelines.
• Your PR follows our pull request guidelines.
• All required CI checks pass before marking the PR ready for review.

[scepter914]

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