Allow users to override non-essential haddock options in a Flavour

We now supply the non-essential options to haddock using the extraArgs field, which can be specified in a Flavour so that if an advanced user wants to change how documentation is generated then they can use something other than the defaultHaddockExtraArgs.

This does have the potential to regress some packaging if a user has overridden extraArgs themselves, because now they also need to add the haddock options to extraArgs. This can easily be done by appending defaultHaddockExtraArgs to their extraArgs invocation but someone might not notice this behaviour has changed.

In any case, I think passing the non-essential options in this manner is the right thing to do and matches what we do for the "ghc" builder, which by default doesn't pass any optmisation levels, and would likewise be very bad if someone didn't pass suitable -O levels for builds.

Fixes #23625 (closed)