[Feature] Add sdk version information to REST User-Agent

### Search before asking

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

### Motivation

PyPaimon REST requests currently use the default `python-requests` User-Agent,
which does not identify the PyPaimon or Python version and makes troubleshooting
client compatibility and upgrades difficult.

### Solution

Use `header.User-Agent`, consistent with Java, and provide these defaults:

- REST: `PyPaimon/ Python/`
- PVFS: `PythonPVFS PyPaimon/ Python/`

Explicitly configured User-Agent values are preserved. Version lookup failures
fall back to `unknown` without affecting requests.

### Anything else?

_No response_

### Are you willing to submit a PR?

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

Start by locating the PyPaimon REST and PVFS request code that constructs User-Agent headers, then trace how SDK and Python versions are obtained. Verify that the specified defaults, explicit User-Agent preservation, and unknown-version fallback work without affecting requests, using the repository's existing REST or PVFS tests if available.

Written by the indexing model from the issue text.