Allow discover extensions to return the manifest and not just the path

### Summary of the new feature / enhancement

Currently, [discover extensions](https://github.com/PowerShell/DSC/blob/main/lib/dsc-lib/src/extensions/discover.rs#L38-L43) are required to return the path to an extension manifest.  Instead, we can allow extensions to return the manifest itself.

### Proposed technical implementation details (optional)

_No response_