Add attoparsec-aeson dependency

[9999years]

This makes servant buildable with the upcoming aeson-2.2 release.

[tchoutri]

@9999years Hi! Is there a reason why you wish to add attoparsec-aeson to the dependencies of servant? :)

[9999years]

Yes, it's for the upcoming aeson-2.2 release, which moves the Data.Aeson.Parser module from aeson to attoparsec-aeson.

src/Servant/API/ContentTypes.hs:79:1: error:
    Could not find module ‘Data.Aeson.Parser’
    Use -v (or `:set -v` in ghci) to see a list of the files searched for.
   |
79 | import           Data.Aeson.Parser
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...

aeson-2.2 isn't released yet, so we don't need to merge this yet, but I'm working on supporting it internally and created this PR to reference in our build.

[tchoutri]

I see, thank you :)

[ysangkok]

Thanks for the contribution, but we merged #1695 instead, so this is no longer necessary.