fix: adjust generators directives with bool flags and add conditional logic for header template

[anGie44]

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Relates #20000

Notes:

• major changes here include:
  • appstream/generate.go: a directive for ListPages generation added to correctly match methods already present
  • directives in <service>/generate.go and <service>/list_pages_gen.go use boolean flags where the string "yes" was previously passed in to simplify usage; docs have been updated as well with the new usage
  • directives in <service>/generate.go no longer pass in a tags flag and generate command as they don't seem to influence file generation

Output from acceptance testing:

$ make gen --> returns no diff 
$ make --> installs successfully
$ make lint --> complains about magic number usage and methods not used 🤔 

[YakDriver]

Looks great to me! 🎉

[YakDriver]

Nice! This is a much better way!

[github-actions]

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[github-actions]

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