DX: Block (config) destination shouldn't recalculate block config ID for block translations

Problem/Motivation

Identified while working on Bean Migrate → :

The block config migration destination plugin EntityBlock forces every block config migrations to calculate the plugin and theme destination properties, because the destination ID is returned by a Storage::loadByProperties() call.

This is a big overhead for translation migrations, because:

• The plugin and the theme need to be re-calculated due to the destination plugin's logic.
• The static map of the plugin property should be kept in sync, but for no reason, because:
• Right now, the block plugin ID uses a lookup – but this isn't used at all, due to the destination plugin.

Proposed resolution

For block config translation migrations (at least) it should be possible to specify only the destination block content ID and the actual property translation.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet