S3: enable overriding x-amz-date from signing server

[ViliamKopecky]

Brief description of the changes

AWS S3 SDK using PostObjectV4 (https://docs.aws.amazon.com/aws-sdk-php/v3/api/class-Aws.S3.PostObjectV4.html) doesnt let you set custom date for policy, and sometimes the dates are different about a second, so the better way for me is setting the date from server.

Maybe even better way would be setting the whole awsParams from server.

What browsers and operating systems have you tested these changes on?

macos chrome

Have you written unit tests? If not, explain why.

I didnt find existing matching test for this.

[donbobka]

+1 to enable overriding of any field (not only date)?