[auth] Add Athenz authentication provider

[equanz]

Motivation

Support Athenz authentication at pulsar-client-go.

Modifications

Add Athenz provider into pulsar/internal/auth, and fix to run Provider.Init() in newClient() function.

Verifying this change

• Make sure that the change passes the CI checks.

This change added tests and can be verified as follows:

• Added unit test for Athenz Authentication Provider

(It seems that the project should add integration test into client_impl.go. However, integrating with Athenz isn't simple...)

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (yes)
• The public API: (no)
• The schema: (no)
• The default values of configurations: (no)
• The wire protocol: (no)

Documentation

• Does this pull request introduce a new feature? (yes)
• If yes, how is the feature documented? (not documented)
• If a feature is not documented yet in this PR, please create a followup issue for adding the documentation

[nkurihar]

LGTM

[equanz]

run integration tests

[equanz]

@wolfstudy @merlimat
Could you review this please?

In this PR, fixed parseParams(), and Provider.Init() to run.

[merlimat]

👍

[hrsakai]

run integration tests