Support OAuth2 authentication in Go instance of pulsar functions

### Search before asking

- [X] I searched in the [issues](https://github.com/apache/pulsar/issues) and found nothing similar.

### Motivation

the pulsar go client already supports the OAuth2, it's better to support OAuth2 in the Go instance too

### Solution

_No response_

### Alternatives

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

- [X] I'm willing to submit a PR!

Start by comparing the existing OAuth2 support in the Pulsar Go client with the Go instance of Pulsar Functions authentication path. Identify the relevant entry points and tests, then verify that the Go instance accepts and uses OAuth2 credentials consistently with the client.

Written by the indexing model from the issue text.