pre-proposal: Add capabilities support for AccessorMetadata

[Xuanwo]

For now, we already have AccessorMetadata which carries the accessor's name and scheme.

In this proposal, we plan to add capabilities support in AccessorMetadata, so that users can check whether the underlying accessor support extending features like presign or multipart.

Along with this change, we will return Unsupported errors for not supported operations instead of panic.

[Xuanwo]

Implemented by #409