feat: Automatic Vended Credential Refresh

### Feature Request / Improvement

Today, long-running writes to PyIceberg risk failing when credentials expire mid-transaction. The Java implementation addressed this by adding a vended-credential refresh mechanism that calls a REST catalog endpoint to obtain new credentials whenever the current ones are within five minutes of expiring.

I propose a similar approach for PyIceberg, delivered in three separate PRs:

1. **VendedCredential class implementation** (largest diff) -> [PR](https://github.com/apache/iceberg-python/pull/3507) ready for review
2. **Credential refresh support in `PyArrowFileIO`** -> PR pending review and merge of 1
3. **Credential refresh support in `FsspecFileIO`** -> PR pending review and merge of 1

PRs 2 and 3 depend on PR 1, which provides the core vended-credential implementation.

Start by reviewing PR 3507, which contains the VendedCredential implementation and is the dependency for the remaining work. Then inspect the planned PyArrowFileIO and FsspecFileIO refresh support; the feature is complete when both integrations refresh credentials obtained from the REST catalog before expiry.

Written by the indexing model from the issue text.