Add warnings for private APIs

[miguelgrinberg]

• Renamed the _stability_warning decorator to _availability_warning
• Added a second optional positional argument to it for the Visibility. If omitted it defaults to public.
• If the endpoint is private, the warning reads "This API is private."
• If an API is private and beta or experimental, the warning message for private is used.
• If the visibility is not set or is not private, then the decorator works as before.

[pquentin]

Thanks! LGTM.

[github-actions]

The backport to 8.19 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-8.19 8.19
# Navigate to the new working tree
cd .worktrees/backport-8.19
# Create a new branch
git switch --create backport-3212-to-8.19
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 bbca81a630b4b9bb0fcc26ed98c92fdd612a4387
# Push it to GitHub
git push --set-upstream origin backport-3212-to-8.19
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.19

Then, create a pull request where the base branch is 8.19 and the compare/head branch is backport-3212-to-8.19.

[github-actions]

The backport to 9.1 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-9.1 9.1
# Navigate to the new working tree
cd .worktrees/backport-9.1
# Create a new branch
git switch --create backport-3212-to-9.1
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 bbca81a630b4b9bb0fcc26ed98c92fdd612a4387
# Push it to GitHub
git push --set-upstream origin backport-3212-to-9.1
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-9.1

Then, create a pull request where the base branch is 9.1 and the compare/head branch is backport-3212-to-9.1.

[github-actions]

The backport to 9.2 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-9.2 9.2
# Navigate to the new working tree
cd .worktrees/backport-9.2
# Create a new branch
git switch --create backport-3212-to-9.2
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 bbca81a630b4b9bb0fcc26ed98c92fdd612a4387
# Push it to GitHub
git push --set-upstream origin backport-3212-to-9.2
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-9.2

Then, create a pull request where the base branch is 9.2 and the compare/head branch is backport-3212-to-9.2.