Extend 1Password secret provider to return the entire entry as JSON

[alecthomas]

Currently a single field must be specified as op://<vault>/<entry>/<field>, which is always treated as a single JSON string.

But sometimes we want a composite value. A proposal @mistermoe and I came up with is that if the reference is to the entry itself, eg. op://<vault>/<entry>, FTL would retrieve all fields as a JSON object.

eg. Given the following entry as JSON:

{
  "id": "xyz",
  "title": "JSON",
  "tags": ["json"],
  "version": 2,
  "vault": {
    "id": "xyz",
    "name": "Personal"
  },
  "category": "LOGIN",
  "last_edited_by": "xyz",
  "created_at": "2024-05-15T19:12:04Z",
  "updated_at": "2024-05-15T19:19:54Z",
  "additional_information": "test",
  "fields": [
    {
      "id": "password",
      "type": "CONCEALED",
      "purpose": "PASSWORD",
      "label": "password",
      "value": "aSecretPassword",
      "reference": "op://Personal/JSON/password",
      "password_details": {
        "strength": "VERY_GOOD"
      }
    },
    {
      "id": "username",
      "type": "STRING",
      "purpose": "USERNAME",
      "label": "username",
      "value": "test",
      "reference": "op://Personal/JSON/username"
    },
    {
      "id": "notesPlain",
      "type": "STRING",
      "purpose": "NOTES",
      "label": "notesPlain",
      "reference": "op://Personal/JSON/notesPlain"
    }
  ]
}

If the secret reference was op://Personal/JSON/password we would return the JSON string quoted value "aSecretPassword".

If the secret reference was op://Personal/JSON we would retrieve the JSON object {"password": "aSecretPassword", "username": "test"}. Note that notesPlain would be omitted because it has no value.

Unfortunately we'll need to manually parse the op:// reference in order to pass the individual values to op item get --json, because it doesn't support op:// references directly. We'll then need to unmarshal the JSON into an intermediate structure in order to retrieve the fields.

[alecthomas]

This will resolve #1494

[mistermoe]

@gak thank you!