Output a redirect map when merging multipath operations

[swallez]

Follow-up to #4415: optionally output a redirect map when merging multipath operations.

This adds a --multipath-redirects to the conversion CLI and to the make transform-to-openapi-for-docs Make target.

This produces a <openapi-file>.redirects.csv alongside the <openapi-file>.json. It's a CSV file containing (old-url -> target-url) mappings, e.g.:

...
indices-get-alias-1,indices-get-alias
indices-get-alias-2,indices-get-alias
indices-get-alias-3,indices-get-alias
indices-get-data-stream-1,indices-get-data-stream
indices-get-field-mapping-1,indices-get-field-mapping
indices-get-index-template-1,indices-get-index-template
indices-get-mapping-1,indices-get-mapping
indices-get-settings-1,indices-get-settings
...

The values are the last part of the URLs, e.g. https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-indices-get-alias-1, which should make it easy to build a redirect map. We can change it if another format is more convenient.

[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-4545-to-9.0
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 6132f6c1e57bac59225645e067d277483456b0fa
# Push it to GitHub
git push --set-upstream origin backport-4545-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-4545-to-9.0.