IBindContext.RegisterObjectParam needs [Const] on the PWSTR in parameter

RegisterObjectParam has a PWSTR parameter but it should be a [Const] param so we can promote the parameter to be a string in the projections.

```
[Documentation("https://learn.microsoft.com/windows/win32/api/objidl/nf-objidl-ibindctx-registerobjectparam")]
HRESULT RegisterObjectParam([In] PWSTR pszKey, [In] IUnknown punk);
```

pszKey should also have `[Const]`.