Write relations directly as part of batch add / update of entities.

This implies a slight change of the CommonDatabase contract.

The huge LDAP dataset is now down to 10s to write, both on postgres and sqlite.

.changeset/purple-cycles-switch.md

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+---
+'@backstage/plugin-catalog-backend': minor
+---
+
+Write relations directly as part of batch add / update of entities.
+
+Slight change of the `CommonDatabase` contract:
+
+## `addEntity` removed
+
+This method was unused by the core, and rendered unnecessary when `addEntities`
+exists.
+
+If you were a user of `addEntity`, please call `addEntities` instead, with an
+array of one element.
+
+## `DbEntityRequest` has a new field `relations`
+
+This is the structure that is passed to `addEntities` and `updateEntity`. It
+used to be the case that you needed to call `setRelations` separately, but now
+this instead happens directly when you call `addEntities` or `updateEntity`.
+
+If you were using `addEntities` or `updateEntity` directly, please adapt your
+code to add the `relations` array to each request. If you were calling
+`setRelations` separately next to these methods, you no longer need to do so,
+after adding the relations to the `DbEntityRequest`s.