[File system] Allow users to configure STS endpoint for S3 file system

### Search before asking

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

### Motivation

Allow users to explicitly configure the [STS endpoint](https://docs.aws.amazon.com/general/latest/gr/sts.html) that should be used.

### Solution

- Add the following config options for the S3 file system

- `fs.s3a.sts.endpoint`: The endpoint URL of the AWS Security Token Service to use for authenticating to S3.
- `fs.s3a.sts.region`: The region to use for SigV4 signing of requests. If not set, `fs.s3a.region` will be used.

- If `fs.s3a.sts.endpoint` is set, build the STS client in `src/main/java/com/alibaba/fluss/fs/s3/token/S3DelegationTokenProvider.java` with the desired endpoint configuration.

### Anything else?

_No response_

### Willingness to contribute

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

Start with src/main/java/com/alibaba/fluss/fs/s3/token/S3DelegationTokenProvider.java and trace how the S3 STS client is built. Add support for fs.s3a.sts.endpoint and fs.s3a.sts.region, including fallback to fs.s3a.region, and verify that configured values affect endpoint selection and SigV4 signing.

Written by the indexing model from the issue text.