feat(NbDialogService): support passing values to input/model signals

[junglerobba]

This makes an exception for input and model signals in the DialogConfig context, so the typehint will show as the type of the signal's value, but internally it will create a signal with the passed value. Technically these will just be readonly signals instead of input or model signals, but since components used as a dialog do not support regular outputs anyway, it should not matter. When used in a regular context the input signal will not be overwritten and work as usual.

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• [x] I read and followed the CONTRIBUTING.md guide.

Short description of what this resolves:

closes https://github.com/akveo/nebular/issues/3256 (I would not classify this as a bug but a missing feature instead)