feat(auth0-auth-js): add support for environment variables

[frederikprijck]

Description

Using ENV Variables is not uncommon, so we want some more built-in support for them, especially for those things that are most likely to differ between environments.

This PR adds support for the following environment variables:

• AUTH0_DOMAIN - Your Auth0 tenant domain
• AUTH0_CLIENT_ID - Your application's client ID
• AUTH0_CLIENT_SECRET - Your application's client secret (optional - depending on the authentication method you want to use)
• AUTH0_CLIENT_ASSERTION_SIGNING_KEY - Your application's client assertion signing key (optional - depending on the authentication method you want to use)

When both constructor options and environment variables are configured, the constructor option takes precedence.

References

N/A

Testing

• This change adds test coverage for new/changed/fixed functionality

Checklist

• I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• All active GitHub checks for tests, formatting, and security are passing
• The correct base branch is being used, if not the default branch