changesets: added changeset for ScmAuthApi

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

.changeset/purple-scissors-allow.md

@@ -0,0 +1,44 @@
+---
+'@backstage/integration-react': patch
+---
+
+Added `ScmAuthApi` along with the implementation `ScmAuth`. The `ScmAuthApi` provides methods for client-side authentication towards multiple different source code management services simultaneously.
+
+When requesting credentials you supply a URL along with the same options as the other `OAuthApi`s, and optionally a request for additional high-level scopes.
+
+For example like this:
+
+```ts
+const { token } = await scmAuthApi.getCredentials({
+  url: 'https://ghe.example.com/backstage/backstage',
+  additionalScope: {
+    repoWrite: true,
+  },
+});
+```
+
+The instantiation of the API can either be done with a default factory that adds support for the public providers (github.com, gitlab.com, etc.):
+
+```ts
+// in packages/app/apis.ts
+ScmAuth.createDefaultApiFactory();
+```
+
+Or with a more custom setup that can add support for additional providers, for example like this:
+
+```ts
+createApiFactory({
+  api: scmAuthApiRef,
+  deps: {
+    gheAuthApi: gheAuthApiRef,
+    githubAuthApi: githubAuthApiRef,
+  },
+  factory: ({ githubAuthApi, gheAuthApi }) =>
+    ScmAuth.merge(
+      ScmAuth.forGithub(githubAuthApi),
+      ScmAuth.forGithub(gheAuthApi, {
+        host: 'ghe.example.com',
+      }),
+    ),
+});
+```