[SDK-3653] Add proxy support to Management Client

[adamjmcgrath]

Changes

Pass proxy param to rest-facade client to enable proxy support

const { ManagementClient } = require('auth0');

const client = new ManagementClient({
  token: '{YOUR_API_V2_TOKEN}',
  domain: '{YOUR_ACCOUNT}.auth0.com',
  proxy: 'https://myproxy.com:1234'
});

References

See https://github.com/ngonzalvez/rest-facade#proxy-support
Fixes #613

Testing

• This change adds unit test coverage

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All existing and new tests complete without errors