feat(material/dialog): MatDialog: change member _dialog (cdk) from private to protected (#28019)

[dolezaljan]

Having customized MatDialog allow to have customized cdk/dialog inside of it.

c.f. #28019 for details

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[crisbeto]

LGTM

[dolezaljan]

ad merge: fix commit messge tag:

feat(material/dialog): change member _dialog (cdk) from private to protected (#28019)
    
Allow to have custom cdk/dialog as a member of custom MatDialog.
Example usage: scoped dialogs.
    
Closes #28019

I can push this one if desirable.

[crisbeto]

It's okay, I'll reword it when merging in the PR.

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