Add uploaded_at field in package api

[Kleidukos]

At present time, the information returned by the Package JSON API amounts to:

{
    "author": "Hécate Moonlight",
    "copyright": "",
    "description": "The 'Display' typeclass provides a solution for user-facing output that does not have to abide by the rules of the Show typeclass.",
    "homepage": "https://github.com/haskell-text/text-display#readme",
    "license": "MIT",
    "metadata_revision": 0,
    "synopsis": "A typeclass for user-facing output"
}

This PR aims to implement support for the package upload timestamp in this payload.

The final result is this:

{
    "author": "Hécate Moonlight",
    "copyright": "",
    "description": "The 'Display' typeclass provides a solution for user-facing output that does not have to abide by the rules of the Show typeclass.",
    "homepage": "https://github.com/haskell-text/text-display#readme",
    "license": "MIT",
    "metadata_revision": 0,
    "synopsis": "A typeclass for user-facing output",
    "uploaded_at": "2022-05-22T22:24:48.997120639Z"
}

[Kleidukos]

@gbaz I tried to make the changes as less invasive as possible. Is there something that should be improved?

[andreasabel]

Looks reasonable!, just cosmetical changes requested.

[Kleidukos]

Annex question: I'm not super clear on how information is stored, and especially the download numbers. I'd like to expose that kind of information in a subsequent PR.

[andreasabel]

Great, thanks!

[Kleidukos]

@gbaz soft-ping

[andreasabel]

This PR looks reasonable and I do not expect it to break something, so I dared to merge it. @gbaz, please revert if you disagree.