Update signing logic for correct custom token format

[kosukesaigusa]

Background

Create custom token here:

final adminApp = FirebaseAdminApp.initializeApp(
  'project-id',
  Credential.fromServiceAccountParams(
    clientId: 'client-id',
    privateKey: 'private-key',
    email: 'email',
  ),
);
final auth = Auth(adminApp);
final customToken = await auth.createCustomToken('some-user-id');
print('customToken: $customToken');

Then, try to use the custom token to sign in Firebase Auth with on Flutter client app:

final userCredential = await FirebaseAuth.instance.signInWithCustomToken(customToken);

The exception like the following is thrown:

FirebaseAuthException ([firebase_auth/invalid-custom-token] The custom token format is incorrect. Please check the documentation.)

If I use the custom token created by JS (TS) SDK by the same project service account on my Flutter client app, it successfully signs in.

admin.initializeApp({
    credential: admin.credential.cert(serviceAccount),
    databaseURL: `https://${serviceAccount.projectId}.firebaseio.com`
})

const main = async () => {
    const customToken = await admin.auth().createCustomToken(`some-user-id`)
    console.log(`customToken: ${customToken}`)
}

main()

I noticed the length of custom tokens are different from each other.

# Example of created custom token by this package:
eyJ*****************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************z0A

# Example of created custom token by JS (TS) SDK:
eyJ****************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************uFA

Then this PR fixes the RSA-SHA256 signing logic using pointycastle package instead of crypto package.

After the change,

• The length of created custom tokens is the same as the ones created by JS (TS) SDK.
• Successful signing with custom token is confirmed.

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I am so excited by and curious about this project, enabling us to write Firebase Admin SDK server-side code by Dart!

Thank you so much for developing such a wonderful project!