Expand a new external var tag

[jiaqizho]

Change logs

External toast in CBDB have the fixed structure, the vartag_external used to tell which way to detoast it.

If you want to add an external toast implementation in the extension without changing the kernel, then you need to add a new tag in vartag_external.

The current change defines an extension generic tag which named VARTAG_CUSTOM, this kind of tag is not used in the kernel, which means that the datum returned from the extension should not be a toast with this kind of tag. This tag is only used within the extension.

Why are the changes needed?

nope

Does this PR introduce any user-facing change?

nope

How was this patch tested?

nope

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🥳