Enhancement request: `--preview-json` to read previews from a simple key-value object instead of spawning an external process

PSFzf uses fzf to power tab-completion menus in PowerShell. Tab completions in PowerShell are rich objects generated in-memory with a few fields:
`CompletionText`, `ListItemText`, `ResultType`, `ToolTip`

`ToolTip` often contains useful metadata. For example, for commands, it contains usage information:
*Here is a screenshot of PSReadLine's `MenuCompletion` which shows completion entries as an interactive menu. No fuzzy matching though :( The ToolTip string is the multiline blue text*

To expose these tooltips in the preview window, we need to provide a process that can look up each tooltip by name, but all the info is already in-memory. Could there be some way to pass this data structure to `fzf` either to stdin, or via a temp file referenced by `--preview-json-file` or similar?

No files, tests, or implementation entry points are named. Start by clarifying how fzf should accept the in-memory PowerShell completion fields, including whether stdin or a temporary file is intended; done means preview tooltips can be shown without spawning an external lookup process.

Written by the indexing model from the issue text.