Support for custom NavigationBar

[maheshj01]

This issue is similar to https://github.com/maheshmnj/navbar_router/issues/8

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

Although NavbarRouter adds support for different NavigationBars going forward, users have different design specs for their apps which prevents them from using this package. We should Add support for a custom NavigationBar which users should implement from NavbarBase and pass to NavbarRouter this will allow users to add their own NavigationBar and yet use all the features of this package.

Describe the solution you'd like Perhaps we should add a NavbarRouter.custom constructor which also takes in NavBarBase as a parameter to support this request.