changesets: add changeset for ConfigSource addition

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/moody-lizards-beam.md

@@ -0,0 +1,40 @@
+---
+'@backstage/config-loader': minor
+---
+
+Introduced a new config source system to replace `loadConfig`. There is a new `ConfigSource` interface along with utilities provided by `ConfigSources`, as well as a number of built-in configuration source implementations. The new system is more flexible and makes it easier to create new and reusable sources of configuration, such as loading configuration from secret providers.
+
+The following is an example of how to load configuration using the default behavior:
+
+```ts
+const source = ConfigSources.default({
+  argv: options?.argv,
+  remote: options?.remote,
+});
+const config = await ConfigSources.toConfig(source);
+```
+
+The `ConfigSource` interface looks like this:
+
+```ts
+export interface ConfigSource {
+  readConfigData(options?: ReadConfigDataOptions): AsyncConfigSourceIterator;
+}
+```
+
+It is best implemented using an async iterator:
+
+```ts
+class MyConfigSource implements ConfigSource {
+  async *readConfigData() {
+    yield {
+      config: [
+        {
+          context: 'example',
+          data: { backend: { baseUrl: 'http://localhost' } },
+        },
+      ],
+    };
+  }
+}
+```