Add rerank task type specification

[timgrein]

Adding rerank task type specification for the elastic provider

Closes #4609

[timgrein]

@pquentin spec PR for elastic/elasticsearch#126022 (can't add you as reviewer, therefore the comment ping)

Edit: could you also add the appropriate labels for 9.1/8.19? I don't seem to have the corresponding rights

[l-trotta]

hey @timgrein thanks for contributing! could you please rebase this onto main and regenerate the output?

[pquentin]

I just realized that EisTaskType is unused. Not sure what the fix is here.

[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-4488-to-8.19
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 7c4997b914e6737c2986b71dfa0b4079de6b5707
# Push it to GitHub
git push --set-upstream origin backport-4488-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-4488-to-8.19.

[github-actions]

The backport to 9.0 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-9.0 9.0
# Navigate to the new working tree
cd .worktrees/backport-9.0
# Create a new branch
git switch --create backport-4488-to-9.0
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 7c4997b914e6737c2986b71dfa0b4079de6b5707
# Push it to GitHub
git push --set-upstream origin backport-4488-to-9.0
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-9.0

Then, create a pull request where the base branch is 9.0 and the compare/head branch is backport-4488-to-9.0.