`CustomPage` and `Paged` has the same friendly name

The `CustomPage` model ([here](https://github.com/Azure/typespec-azure/blob/367549dd204b6fd78c7a64fcca3cfd360b0a31ed/packages/typespec-azure-core/lib/foundations.tsp#L243)) and `Paged` model ([here](https://github.com/Azure/typespec-azure/blob/367549dd204b6fd78c7a64fcca3cfd360b0a31ed/packages/typespec-azure-core/lib/models.tsp#L15)) have exactly the same `friendlyName`, this leads to models with the same name in the SDK and might lead to issues.

Taking a closer look, `Page` is identical to `CustomPage` when the second argument is empty.
Therefore how about this for a fix:
We change `Page` as an alias of `CustomPage`:
```typespec
alias Page = CustomPage;
```
which in the functionality's perspective, should not break anything.

Start by comparing the CustomPage definition in packages/typespec-azure-core/lib/foundations.tsp and the Paged definition in packages/typespec-azure-core/lib/models.tsp, including their friendlyName values and the Page relationship. The change is done when the duplicate friendly name is removed without breaking the generated SDK model behavior.

Written by the indexing model from the issue text.