[Reporting] Move the core parts of the code to src/plugins

[tsullivan]

TBD: state clearly the goals of refactoring

Parts of Reporting are available with a Basic license. By moving parts of Reporting to src/plugins, OSS applications would be able to register their own integration points with the Reporting plugin, instead of having Reporting code provide those registrations. Doing this will simplify the Reporting plugin code. To achieve this, parts of Reporting code should move to src/plugins:

• ReportingCore
• ExportTypeRegistry
• CSV ExportType definitions

Other parts of Reporting code should remain in xpack:

• PNG export
• PDF export
• get_screenshots function

Depends on other parts of Kibana moving to src/plugins:

• License plugin
• Task Manager
• Security plugin

[elasticmachine]

Pinging @elastic/kibana-reporting-services (Team:Reporting Services)

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServices)

[tsullivan]

Possibly intersects with #81762

[tsullivan]

@jloleysens and I discussed this and he noted that Reporting has a dependency on Task Manager, and that stands in the way of moving any part of Reporting to src/plugins.

We also discussed the fact that there is a major part of Reporting code that has a Platinum license, and these features are connected to OSS features. Therefore, there still needs to be some kind of separate connecting piece that registers the Kibana features on behalf of OSS features. That idea goes back to the argument for an unpopular proposal I made earlier: #94995

We ended on the thought that if we can only move Basic license code to src/plugins, it undermines the argument to move Reporting to src/plugins because half the code is above Basic license.

[tsullivan]

Pretty much all of the code within Reporting is x-pack specific. We should simplify individual parts of Reporting by breaking it up, but we don't necessarily need to move parts into src/plugins unless there is a benefit.