Improve child performances

[delvedor]

The client code has been refactored to speed up the performances of the child method.
Before this pr, creating many children per second would have caused a high memory consumption and a spike in CPU usage.
This pr changes the way the client is created by refactoring the code generation, now the clients methods are no longer added to the instance with a for loop but via prototypal inheritance. Thus, the overall performances are way better, now creating a child is ~5 times faster, and it consumes ~70% less memory.

This change should not cause any breaking change unless you were mocking the client methods. In such case you should refactor it, or use elasticsearch-js-mock.

Finally, this change should also fix once and of all the bundlers support.

cc @kobelb @roncohen @nchaulet
Closes: #1232 #1290

[github-actions]

The backport to 7.x 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-7.x 7.x
# Navigate to the new working tree
cd .worktrees/backport-7.x
# Create a new branch
git switch --create backport-1314-to-7.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick ---mainline 1 a064f0f357ea5797cb8a784671b85a6b0c88626d
# Push it to GitHub
git push --set-upstream origin backport-1314-to-7.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-7.x

Then, create a pull request where the base branch is 7.x and the compare/head branch is backport-1314-to-7.x.