Update cluster:monitor/main with transport version info

[thecoop]

Currently cluster:monitor/main (implemented by MainResponse) returns a json object with a minimum_wire_compatibility_version field, currently the node version. This should be changed to the transport version, and a new field added for the node version.

[elasticsearchmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[thecoop]

What are the compatibility restrictions around this? I can't find any documentation on this output format, can we change it freely?

[DaveCTurner]

It's subject to the same BwC concerns as all other APIs. It's definitely used by e.g. clients to make decisions about which features they should/shouldn't use. So we will need to leave minimum_wire_compatibility_version fixed at "7.17.0" throughout the 8.x series, but we can theoretically change it or remove it in 9.x as long as we do the deprecation dance.

Edit: but, as with other APIs, we do not consider adding fields to the response a breaking change so we can do that freely.