[SDK-3541] Rework ignoreCache to cacheMode and introduce 'cache-only'.

[frederikprijck]

Changes

This PR reworks ignoreCache to use cacheMode instead, allowing for three caching modes.

• on (default): read from the cache caching, but fall back to Auth0 as needed
• off: don’t read from the cache
• cache-only: read from the cache, don’t fall back to Auth0

With the new cache-only option, we provide the user with the ability to try and retrieve a token from the cache, but not call Auth0 when there isn't any token to return, or the token is expired.

Users that used to use ignoreCache: true, should now use cacheMode: 'off' instead.

Testing

• This change adds unit test coverage
• This change adds integration test coverage
• This change has been tested on the latest version of the platform/language

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All code quality tools/guidelines have been run/followed