Fix credential item category and add another credential fields

[speto]

I believe part of the problem with the credential category lies in if f.ID == "credential" && item.Category == "API_CREDENTIAL" where "credential" is checked against f.ID.

I'm not sure if I'm following correctly everything, but I believe the correct f field for this comparison should be Label not ID. Leaving it like rest of the fields on 350-362 should be sufficient.

I followed discussion in the original PR but I don't feel much clearer on it. Just think this shouldn't have been merged without tests.

So, first I added a test for credential category, which successfully failed. Then I tried fixing the issue and discovered the f.ID problem. Afterwards I added additional credential fields based on what 1password returns for that category and updated the docs. The type field is shared between both database and credential category.

This is also related to #52

[github-actions]

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