Fix updater value buffer isolation

[Daniiiil1]

Summary

• move the diagnostic key-value buffer from file-static storage into each diagnostic_updater_t instance
• set the ROS sequence capacity in elements rather than bytes
• add a regression test that initializes two updaters and verifies their buffers cannot alias or overwrite each other
• add the contributor entry required by CONTRIBUTING.md

Testing

• ament uncrustify configuration check passed for the changed C, C++, and header files
• git diff --check
• the added gtest is exercised by the repository Humble and Rolling CI matrix

Fixes #39