Enhancement: add sha256 field for release

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

The other half of: https://github.com/python/release-tools/pull/310

### Describe the solution you'd like

Add sha256 to ReleaseFiles

### Additional context
https://github.com/python/pythondotorg/issues/1227#issuecomment-3564239648