Support DefaultText

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close urfave/cli#2210
close #9

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If it is squashmerged I let it as is ... else I will rebase stuff

plz tell me :)

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@dearchap refactored the code to be more streamline and different doc generation will use the same func to get default falue e.g. no extra for bool flags outside of getFlagDefaultValue()

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this also matches the behaviour we have from --help more ..:
example:

user@laptop[woodpecker] # ./dist/woodpecker-agent --help | grep -E 'false|true'
   --grpc-secure              should the connection ... a secure transport (default: false) [$WOODPECKER_GRPC_SECURE]
   --grpc-skip-insecure       should the grpc server certificate be verified, ... (default: true) [$WOODPECKER_GRPC_VERIFY]
   --healthcheck              enable healthcheck endpoint (default: true) [$WOODPECKER_HEALTHCHECK]
   --pretty                   enable pretty-printed debug output (default: false) [$WOODPECKER_DEBUG_PRETTY]
   --nocolor                  disable colored debug output, ... (default: true) [$WOODPECKER_DEBUG_NOCOLOR]
...

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thanks