Add C++ and Python example for OAuth2 with scope field

[BewareMyPower]

These two PRs have been cherry-picked to branch-2.8 and included in 2.8.1.10 release.

• https://github.com/apache/pulsar/pull/12305 introduces the support for scope field in OAuth2 authentication.
• https://github.com/apache/pulsar/pull/12341 fixes the incorrect implementation of OAuth2 authentication for some OAuth2 provider like Azure.

This PR adds the related examples for C++ and Python client.

In addition, this PR introduces a better way to configure OAuth2 params for C++ client by passing a ParamMap object.