pre-proposal: Add presign support

[Xuanwo]

As requested by #394, OpenDAL will add presign support so that our users can generate a resigned request to users without leaking the secret key.

Proposed changes:

presign will become a new capability as described in #404

• Add presign support in Accessor: presign(path: &str, method: http::Method, headers: http::Headers, expires: Duration) -> SignedRequest
• Expose public API via Operator: presign_{read,delete}with the same args.
  • write will not have content
  • list will not supported in this proposal

[Xuanwo]

Is size in write required?

[Xuanwo]

Only required during request, not in presign.