Prepare JS client 9.0

[JoshMock]

• generate alpha that drops typesWithBody for Kibana team to try out
• change default timeout/keepalive settings to reflect Elasticsearch best practices
• publish as ESM, either by default or as an alternative option
• migrate type definitions into their relevant API files (still TBD if effort is worth the value)
• write codemods to support migration away from body prop and any other breaking changes
• get all docs up to date with the Markdown conversion

[JoshMock]

I've published a 9.0.0-alpha.1 release to npm that drops body param support, and have passed that info to the Kibana core team to help them start planning their upgrade to the 9.0 client.

That client upgrade may not happen until after Kibana 9.0 is released, just given the amount of work it would require before the upcoming feature freeze.

[JoshMock]

Dropping default 30-second timeout on all HTTP requests is done in elastic/elastic-transport-js#201.

[JoshMock]

Client changes necessary to support dropping the default timeout value are in #2573.

[JoshMock]

Changes to the original plan to drop the body parameter are being discussed with the Elastic breaking changes committee, in part due to the discovery documented in #2584.

[JoshMock]

This work is currently blocked by #2605

[JoshMock]

#2605 is resolved, and the changes necessary to resolve #2584 has been merged and published in 9.0.0-alpha.4.

With that change merged, codemods are probably not a necessary thing to provide for the 8-to-9 client migration, since the impact of the breaking change is now much smaller.