chore!: Eliminate public Foundation.Date extension

[jbelkins]

Description of changes

• Eliminates the creation by typealias of a ClientRuntime-specific Date type. ClientRuntime doesn't need its own Date.
• Eliminates the public extension on Foundation.Date since such extensions on other modules' types can result in name collisions.

aws-sdk-swift PR awslabs/aws-sdk-swift#1947 depends on this change. Since the logic in the deleted extension is only ever used in one place, it is reproduced inline when needed in aws-sdk-swift.

This change is breaking because it eliminates the ClientRuntime.Date type and the .withoutFractionalSeconds() extension. Customer impact should be zero since these types are only used in non-customer-facing components.

Scope

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.