Fix ScoreFunction: variants are optional

[swallez]

The weight function score is not exactly a function type even if documented as such, but a property common to all function types. Now weight can also be used standalone, meaning that a ScoreFunction union can exist without an actual function being set, and is valid with the weight property set.

This PR allows the ScoreFunction union to be built without choosing a variant, and also removes the unnecessary parent ScoreFunction parent class (see elastic/elasticsearch-specification#1666).

Fixes #144, fixes #113, fixes #86

[github-actions]

The backport to 7.17 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-7.17 7.17
# Navigate to the new working tree
cd .worktrees/backport-7.17
# Create a new branch
git switch --create backport-257-to-7.17
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 8b6b6b00d0e9f6b853dec09aef33ea1760cb3c29
# Push it to GitHub
git push --set-upstream origin backport-257-to-7.17
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-7.17

Then, create a pull request where the base branch is 7.17 and the compare/head branch is backport-257-to-7.17.

[github-actions]

The backport to 8.2 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-8.2 8.2
# Navigate to the new working tree
cd .worktrees/backport-8.2
# Create a new branch
git switch --create backport-257-to-8.2
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 8b6b6b00d0e9f6b853dec09aef33ea1760cb3c29
# Push it to GitHub
git push --set-upstream origin backport-257-to-8.2
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.2

Then, create a pull request where the base branch is 8.2 and the compare/head branch is backport-257-to-8.2.