changesets: added changesets for auth-backend refactor

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

.changeset/dry-wasps-occur.md

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+---
+'@backstage/plugin-auth-backend-module-gcp-iap-provider': minor
+---
+
+New module for `@backstage/plugin-auth-backend` that adds a GCP IAP auth provider.

.changeset/healthy-tools-count.md

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+---
+'@backstage/plugin-auth-backend': patch
+---
+
+Deprecated several exports that are now available from `@backstage/plugin-auth-node` instead.

.changeset/pink-cups-rescue.md

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+---
+'@backstage/plugin-auth-node': minor
+---
+
+Introduced a new system for building auth providers for `@backstage/plugin-auth-backend`, which both increases the amount of code re-use across providers, and also works better with the new backend system.
+
+Many existing types have been moved from `@backstage/plugin-auth-backend` in order to avoid a direct dependency on the plugin from modules.
+
+Auth provider integrations are now primarily implemented through a pattern of creating "authenticators", which are in turn specific to each kind of integrations. Initially there are two types: `createOAuthAuthenticator` and `createProxyAuthenticator`. These come paired with functions that let you create the corresponding route handlers, `createOAuthRouteHandlers` and `createProxyAuthRouteHandlers`, as well as provider factories, `createOAuthProviderFactory` and `createProxyAuthProviderFactory`. This new authenticator pattern allows the sign-in logic to be separated from the auth integration logic, allowing it to be completely re-used across all providers of the same kind.
+
+The new provider factories also implement a new declarative way to configure sign-in resolvers, rather than configuration through code. Sign-in resolvers can now be configured through the `resolvers` configuration key, where the first resolver that provides an identity will be used, for example:
+
+```yaml
+auth:
+  providers:
+    google:
+      development:
+        clientId: ...
+        clientSecret: ...
+        signIn:
+          resolvers:
+            - resolver: emailMatchingUserEntityAnnotation
+            - resolver: emailLocalPartMatchingUserEntityName
+```
+
+These configurable resolvers are created with a new `createSignInResolverFactory` function, which creates a sign-in resolver factory, optionally with an options schema that will be used both when configuring the sign-in resolver through configuration and code.
+
+The internal helpers from `@backstage/plugin-auth-backend` that were used to implement auth providers using passport strategies have now also been made available as public API, through `PassportHelpers` and `PassportOAuthAuthenticatorHelper`.

.changeset/sweet-hairs-complain.md

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+---
+'@backstage/plugin-auth-backend-module-google-provider': minor
+---
+
+New module for `@backstage/plugin-auth-backend` that adds a Google auth provider.