feat: add configuration for using persistent connections

[alaczi]

Changes

Exposing the underlying library's - (rest-facade) options for keepAlive connections

We experienced a lot of ETIMEDOUT errors during our load tests and keeping alive connections mitigated the errors.

Note: more work needed for auth/TokensManager and auth/UsersManager as that is using axios (static way) to do the calls. To be able to use keepAlive there an axios instance need to be created and reused in the requests.

References

https://github.com/auth0/node-auth0/issues/545

Testing

Please describe how this can be tested by reviewers. Be specific about anything not tested and reasons why. If this library has unit and/or integration testing, tests should be added for new functionality and existing tests should complete without errors.

• [x] This change adds unit test coverage
• [ ] This change adds integration test coverage

Checklist

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

[adamjmcgrath]

Thanks for raising this @alaczi - I'll take a look at it early next week