support multiple Android SHA256 fingerprints configuration in firebase extension

[rajda]

Need to support different versions of an app, such as debug / firebase appTester / google play version. Each version is signed or may be signed with a different key and we are able to set only one of them as sha256 fingerprint in FlowLinks Extension configuration at the time (Android SHA256 Fingerprint (Optional) corresponds to sha256_cert_fingerprints in .../.well-known/assetlinks.json).

As a result, the flowLink works with only one version of the app: either the Firebase App Tester version or the Google Play Store version.