changesets: added changesets for initial experimental backend system

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

.changeset/beige-kiwis-know.md

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+---
+'@backstage/backend-plugin-api': minor
+---
+
+Introduced new package for creating backend plugins using the new alpha backend plugin framework.
+This package is still considered **EXPERIMENTAL** and things will change without warning. Do not use this for production.

.changeset/lazy-steaks-tell.md

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+---
+'@backstage/plugin-catalog-node': minor
+---
+
+Added alpha exports for the new experimental backend system.

.changeset/polite-moose-beam.md

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+---
+'@backstage/plugin-catalog-backend': minor
+---
+
+Export experimental `catalogPlugin` for the new backend system. This export is not considered stable and should not be used in production.

.changeset/short-wolves-applaud.md

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+---
+'@backstage/plugin-scaffolder-backend': minor
+---
+
+Export experimental `scaffolderCatalogExtension` for the new backend system. This export is not considered stable and should not be used in production.

.changeset/swift-plants-fix.md

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+---
+'@backstage/backend-app-api': minor
+---
+
+Add initial plumbing for creating backends using the experimental backend framework.
+
+This package is highly **EXPERIMENTAL** and should not be used in production.