Refactor and improve the credential and token mehanism for Filesystem

### Search before asking

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

### Description

Currently, we use `DynamicTemporaryOssCredentialsProvider` in client to access oss which will retrieve token from server.

Also, in the future, we may introduce a new `CredentialsProvider` like `TokenFileBasedCredentialsProvider` in server side which causes it harder to distinguish them.

Also, we have `OSSSecurityTokenProvider` which is used in server to obtain sts token from oss token endpoint.

But the naming is not intuitive. We should have a more intuitive name to distinguish them.

### Willingness to contribute

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

Start by tracing the client-side DynamicTemporaryOssCredentialsProvider and the server-side OSSSecurityTokenProvider, then inspect how CredentialsProvider is used across client and server. Clarify an intuitive naming scheme that distinguishes these providers and update all affected references; done means the credential and token roles are unambiguous throughout the Filesystem code.

Written by the indexing model from the issue text.