[SPI] Set "Google" as the author on Swift Package Index (#10808)

Updated the Swift Package Index manifest file (.spi.yml) to specify "Google" as the authors of the package.

.spi.yml

@@ -1,3 +1,5 @@
 version: 1
 external_links:
   documentation: "https://firebase.google.com/docs/ios/setup"
+metadata:
+  authors: "Google"