[OpenApi] Add branch option to cli

[l-trotta]

Closes #4628.
This doesn't affect main or 9.x, it's only an old docs system issue, but I'll port this everywhere for consistency.
In the old docs the branch name in ext_docs_url (externalDocs in openapi) is a {branch} placeholder that gets resolved in the rust openapi converter by getting the correct branch value from the schema model. The problem is that the branch/version doesn't exist in the schema model anymore, because it was causing problems with PRs, so right now it always defaults to "current".

This PR gets the version for externalDocs from a CLI argument instead. This is a temporary solution, since the whole logic will get rewritten when the repo is transfered.

So no changes for branches main, 9.1, 9.0, while for 8.18 and 8.19 it will be possible to run:

npm run transform-to-openapi -- --schema output/schema/schema.json --flavor stack --output output/openapi/elasticsearch-openapi.json --branch 8.19

and fix the url issue.

Output preview for 8.19: https://github.com/elastic/elasticsearch-specification/blob/backport-4722-to-8.19/output/openapi/elasticsearch-openapi.json

[lcawl]

This sounds great, thanks! Once it's merged, I'll create a separate PR to add that option to the makefile for the 8.19 and 8.18 branches and update our release processes in case they need to be updated (e.g. right now the "8.19" branch has to point to 8.x docs but that will change once we have live 8.19 docs)