Ensure that third-party new platform plugins can be built externally

[spalger]

The plugin generator has been updated to generate new platform plugins, the new platform plugin build system is in place, and starting with 7.7 third-party plugins built using the new platform will need to be built and optimized before being installed. We need to make sure this works and is ready for wider use.

[elasticmachine]

Pinging @elastic/kibana-operations (Team:Operations)

[spalger]

Related to #61731, which is documenting a user trying to write a plugin using the new platform in 7.6 where the new platform plugin isn't being built because the optimizer isn't noticing it. We should make sure we don't have to ship 7.7.1 in order to make this possible for users looking to upgrade to 7.7

[mshustov]

do you want to have done it in 7.8 or 7.7.1?