Add namespace pg_ext_aux for extension

[gfphoenix78]

New table access method(TAM) may have their own internal auxiliary tables, like AO tables have the visimap table, aoseg files, block directory tables. The auxiliary tale is not real user data. It's used to maintain the data structure of the new TAM. The auxiliary table should be protected by the system, or modifying the auxiliary table, by the user, will corrupt the user table.

This namespace mainly allows the extensions to have their internal auxiliary tables and protects them. The namespace of the auxiliary tables is not allowed to change, at least it's not necessary at the moment.

Contributor's Checklist

Here are some reminders and checklists before/when submitting your pull request, please check them:

• Make sure your Pull Request has a clear title and commit message. You can take git-commit template as a reference.
• Sign the Contributor License Agreement as prompted for your first-time contribution(One-time setup).
• Learn the coding contribution guide, including our code conventions, workflow and more.
• List your communication in the GitHub Issues or Discussions (if has or needed).
• Document changes.
• Add tests for the change
• Pass make installcheck
• Pass make -C src/test installcheck-cbdb-parallel
• Feel free to request cloudberrydb/dev team for review and approval when your PR is ready🥳

[CLAassistant]

All committers have signed the CLA.

[my-ship-it]

LGTM