Allow to decentralise Management API token cache

[KillianHmyd]

Describe the problem you'd like to have solved

As the cache of the Management API's token is local to the execution process we end-up generating a lot of Machine-to-Machine tokens when scaling our application horizontally. Having a way to decentralised the cache would help us to reduce the number of generated tokens.

Describe the ideal solution

Ideally we could pass a method to the token properties to handle ourselves the caching mechanism:

const auth0ManagementClient = new ManagementClient({
  token: () => Promise.resolve('my-token')
  clientId,
  domain,
  audience,
  scope,
});

We can also imagine leveraging on the existing Automatic Management API Token Retrieval features:

const auth0ManagementClient = new ManagementClient({
  tokenProvider: {
     enableCache: true,
     setCache: (token) => saveCachedToken(token),
     getCache: () => getCachedToken()
  }
});

Alternatives and current work-arounds

Currently the only solution I see is to create multiple instance of the ManagementClient to control which token is passed to it.

Additional information, if any

[adamjmcgrath]

Hi @KillianHmyd - thanks for raising this

Happy to proceed with your PR, although you can also just do

const auth0ManagementClient = new ManagementClient({ ... });
auth0ManagementClient.tokenProvider = {
  getAccessToken() {
    return Promise.resolve('my-token');
  }
}

[adamjmcgrath]

Fixed by #793