[http] post-migration cleanup

[pgayvallet]

During https://github.com/elastic/kibana/pull/136105, we identified some http domain related cleanup tasks that we think should be performed after the rest of core's code has been fully migrated:

• rename mockRouter to routerMock in @kbn/core-http-router-server-mocks for naming consistency with our other mocks.
• remove the ExternalUrlConfig export from src/core/server/types.ts (and move the type to common instead of server if necessary)
• check for external usages of ensureRawRequest and see if we should expose this from the public package instead.
• Find a way to stop having external consumers import KibanaResponse, probably by providing a typeguard util from the public package

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)