[REQ][swift5] Add config flag to control Hashable conformance behavior

[fl034]

Is your feature request related to a problem? Please describe.

In v5.1.1 #9166 introduced a new default behavior that adopts Hashable for every model via synthetization. That way custom implementations won't be called anymore.

Describe the solution you'd like

After talking with @4brunu we decided that a config flag to control the behavior in general would be good.
On model level you can always use the x-swift-hashable: true to enable Hashable conformance.

We should discuss if the feature should be enabled or disabled as default.

Describe alternatives you've considered

Multiple alternatives would be possible

• Revert Add Hashable for Struct #9166
• Add documentation for x-swift-hashable (I couldn't find it anywhere)