Add support for BYO certificate with `ConfigureCustomDomain(...)`

### Is there an existing issue for this?

- [x] I have searched the existing issues

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

At the moment the `ConfigureCustomDomain(...)` API assumes that you are going to use managed certificates, but one scenario where we could pre-configure certificates and do a one shot deployment is if we upload the certs into an existing keyvault.

This was mentioned here by @YohanSciubukgia
https://github.com/dotnet/docs-aspire/issues/1864#issuecomment-3213367693

### Describe the solution you'd like

We are probably missing a few elements here before we can implement this. We may need to add certificate support to our KeyVault integration, or perhaps we just support specifying the keyvault certificate RID.

### Additional context

_No response_