new feature: allow inject a customized signer for aws s3

### Feature Description

background: https://github.com/apache/iceberg-rust/issues/506

> It seems we need to implement this signer in iceberg-rust and pass it as a customized signer to opendal.

### Problem and Solution

but currently the signer is a struct `AwsV4Signer`.

we need make the signer in `S3Core` a trait before allowing it customizable.

maybe we can defined a trait called `S3Signer` in opendal, and make `AwsV4Signer` as it's implementation by default.

### Additional Context

_No response_

### Are you willing to contribute to the development of this feature?

- [X] Yes, I am willing to contribute to the development of this feature.

Start by reading the S3Core and AwsV4Signer implementations in OpenDAL, then review the linked iceberg-rust issue for how a custom signer needs to be passed through. Done means S3Core accepts an injectable signer abstraction while AwsV4Signer remains the default implementation.

Written by the indexing model from the issue text.