Add fetchApiRef which implements fetch, plus Backstage token header when available.

This intends to be the basis for other plugins' data fetching needs, so that they can transparently interact with the catalog and other parts of the Backstage ecosystem without explicitly having to deal with authenticating themselves.

Signed-off-by: Fredrik Adelöw <freben@gmail.com>

.changeset/blue-queens-sniff.md

@@ -0,0 +1,7 @@
+---
+'@backstage/app-defaults': patch
+'@backstage/core-app-api': patch
+'@backstage/core-plugin-api': patch
+---
+
+Add `FetchApi` and related `fetchApiRef` which implement fetch, with an added Backstage token header when available.

.changeset/lovely-drinks-kiss.md

@@ -0,0 +1,5 @@
+---
+'@backstage/catalog-client': patch
+---
+
+Add the ability to supply a custom `fetchApi`. In the default frontend app setup, this will use the `fetchApiRef` implementation.