Warn when bundle identifier contains uppercase characters

[lbalmaceda]

Changes

In order to improve DX when the Application ID (android) or Bundle Identifier (iOS) values have uppercase characters, a warning will be printed to the device console. This warning only appears on the non-production environment according to the react-native docs.

This is just a warning! The authorize call will go through regardless of the bundle identifier value.

The warning message is the following:

"The Bundle Identifier or Application ID of your app contains uppercase characters and will be lowercased to build the Callback URL. Check the Auth0 dashboard to whitelist the right URL value."

This PR also rephrases the callback URLs section of the README. (feedback?)

References

React native docs about warnings https://reactnative.dev/docs/debugging#warnings
Resolves #130

Testing

No unit tests, only manual.

• This change adds unit test coverage
• This change has been tested on the latest version of the platform/language or why not

Checklist

• I have read the Auth0 general contribution guidelines
• All existing and new tests complete without errors
• All active GitHub checks have passed