Support custom URLs signing

[remicres]

From what I understand GH-218 provides support for loading assets that require signed URLs (relying on the mysterious undocumented gdal.GetSignedURL() method).
Also, it looks like this is intended to work with private cloud providers (Azure, GS, S3, etc.).

We could add some kind of API Capability that consist in doing some POST requests to some specific endpoint, returning signed URLs of assets. This new API capability could be parameterized with a single endpoint URL.

IMO this would be very helpful for people that do not put their data on big private clouds, and want to offer something similar to PC but with home made solutions.

Any thoughts?