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

[zhoulii]

Search before asking

• I searched in the 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/<version> Python/<version>
• PVFS: PythonPVFS PyPaimon/<version> Python/<version>

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?

• I'm willing to submit a PR!