changesets: added changesets for shared environment

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

.changeset/fast-spoons-relax.md

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+---
+'@backstage/backend-plugin-api': patch
+---
+
+Added `createSharedEnvironment` for creating a shared environment containing commonly used services in a split backend setup of the backend.

.changeset/many-dogs-sin.md

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+---
+'@backstage/backend-defaults': patch
+---
+
+Added support to supply a shared environment to `createBackend`, which can be created using `createSharedEnvironment` from `@backstage/backend-plugin-api`.