Patch to support doNotOverwrite() in SupportingFile

[robig]

@martin-mfg In java class org.openapitools.codegen.SupportingFile exists a method named doNotOverwrite which should be used to indicate a "supporting file which should not overwrite a file of the same name.". However, I could not find an implementation of this anywhere in the Java DefaultGenerator.

This patch is simply implementing this behaviour as described in javadoc of the class I mentioned. If the file exists, and the file will be skipped and corresponding status message is set.
Feel free to modify the patch or change the logged text to a more obvious one using the right terminology.

Thank you for the great work!
Rob

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[wing328]

thanks for the PR. can you please add a test if possible?