core-plugin-api: Add tests and changesets

Co-authored-by: blam <ben@blam.sh>
Co-authored-by: Patrik Oldsberg <poldsberg@gmail.com>
Signed-off-by: Johan Haals <johan.haals@gmail.com>

.changeset/slow-olives-confess.md

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+---
+'@backstage/core-plugin-api': patch
+---
+
+Deprecated `register` option of `createPlugin` and the `outputs` methods of the plugin instance.
+
+Introduces the `featureFlags` property to define your feature flags instead.

.changeset/young-hats-shop.md

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+---
+'@backstage/plugin-cost-insights': patch
+---
+
+Supply featureFlags using featureFlag config option.