Move index-pattern migration to kibana plugin

[mikecote]

This PR moves the index-pattern migration to the kibana plugin. This is needed to allow the new import / export api to work between environments that use different licenses. In general migrations and mappings should be paired together in the same plugin.

I came across the issue while writing the new import API. I exported sample data using the basic license and noticed it failed when using that file in oss mode. The new import API applies migrations and throws an error when the data is migrated to a newer version than the most current migration (in this scenario, undefined).

[elasticmachine]

Pinging @elastic/kibana-platform

[jen-huang]

Code LGTM! Thanks for the knowledge about keeping migrations and plugins in the same place. That's good to know for any future work where a licensed plugin may affect OSS saved objects.

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[chrisdavies]

LGTM. And don't forget maybe to open an issue about having migrations enforce that migrations for a property belong to the same plugin that defines the mappings for that property. That seems reasonable to me.

[mikecote]

No problem @jen-huang, this was a new discovery at the same time. We're going to look at adding enforcements to prevent this in the future.

[mikecote]

Thanks @chrisdavies, I have created #31694 to track that issue.