[aspnetcore] Add processing of Port defined in spec

[etherealjoy]

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Description of the PR

Port defined in the spec is processed so that the server is accessible on a predefined port stated in spec instead of default Kestrel port 5000

Listening on 0.0.0.0 is needed to make the server accessible not only when it is running on host but also inside a docker container.

@mandrean @jimschubert

[etherealjoy]

Should I fix it in DefaultCodegen instead so that the mustache variable is updated for all subclass?

[jmini]

Should I fix it in DefaultCodegen instead so that the mustache variable is updated for all subclass?

The problem is that a lot of existing codegen are doing this there own way (other default values, possibility to override the value with an additional property on the CLI...). It will be difficult to generalize it in DefaultCodegen.