FFprobe for File::aux_metadata disabled

[FelixMcFelix]

Songbird version: next

Rust version (rustc -V): rustc 1.69.0 (84c898d65 2023-04-16)

Serenity/Twilight version: N/A

Description:
Testing on #179 showed that ffprobe-based aux_metadata support is broken on two fronts:

1. serde_aux::prelude::deserialize_option_number_from_string does not correctly map a missing field into None.
2. Executing ffprobe with the default tokio parameters on some (particularly short) files leads to a deadlock, despite Command::output being the recommended tool for doing so.

Currently, the aux_metadata calling point is commented out until these points can be figured out.

Steps to reproduce:
Call aux_metadata on any File.