Make Java Client Authentication Asynchronous

### Search before asking

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

### Motivation

The OAuth2 client's authenticate call is blocking, which is not great for the client, and is bad for the proxy because the proxy authenticates from a netty event loop.

### Solution

In the same way that PIP 97 #12105 make the `AuthenticationProvider` async, we should update the `Authentication` interface.

### Alternatives

_No response_

### Anything else?

This change will likely require a PIP. I am unable to write such a PIP at this time, so I am creating this issue to keep track of this missing feature.

### Are you willing to submit a PR?

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

Start with the Authentication interface and the OAuth2 client's blocking authenticate call, then compare the asynchronous AuthenticationProvider change described in PIP 97 (#12105). Done means defining and implementing the asynchronous client authentication change so authentication no longer blocks a Netty event loop, with the required PIP completed.

Written by the indexing model from the issue text.