Allow some values to be use in templates

[jmini]

Existing templates (here or in the swagger-codegen repository) are using these values:

{{modelPackage}}
{{apiPackage}}
{{invokerPackage}}
{{#sortParamsByRequiredFlag}}..{{/sortParamsByRequiredFlag}}
{{#hideGenerationTimestamp}}..{{/hideGenerationTimestamp}}

They need to be available in the additionalProperties map in order to be consumed from the mustache templates.

In addition, setting the value should be possible using setters. This is usefull when the generators are used directly (no maven plugin or no cli integration). Setter for hideGenerationTimestamp has been added.

invokerPackage is present only in AbstractJavaCodegen. Its value is derived from apiPackage or modelPackage. Current behavior needs to be preserved this is why some regression tests were implemented first in 7ad6ca2.

[HugoMario]

I don't think this could cause an error or something similar, but i'm wondering if you'd need to set modelPackage in additionalProperties if its value is null or empty.

[jmini]

Thank you for your feedback. I have added a is null or empty check.

To stay consistent, I updated all code location where the same pattern is used.

[HugoMario]

awesome, thanks !

[HugoMario]

@jmini
i will need to revert these PR, since is causing problem to php client generator, and i think it'll be present to next languages generators to add.

The best approach here is add those changes in the specific language overriding methods in DefaultCodegenConfig class, instead changing it.

I understand is more work, i can help you with that if you're agree.

[HugoMario]

basically i will only rever this commit and tweaks test related.