Audit Log Change variants are undocumented

[vilgotf]

Whilst discussing #2110 in Discord (https://discord.com/channels/745809834183753828/1070716906098671706) it came up that Discord no longer documents the AuditLogChange variants. We should therefore, as usually, remove the variants ourselves too, transforming it into a struct where new_value and old_value can be of any type.

I can think of three ways to achieve this:

1. Expose the values as serde_json::Value or simd_json::value::owned::Value. Downside: new feature flags for twilight-model.
2. Expose the values as serde_value::Value (which is already a dependency). Downside: awkward to use for users.
3. Expose the values as raw JSON (of type String) by deserializing and reserializing them back to strings (trickier than it seems to optimize out). This is what Remora does, as can be seen here Remora/Remora.Discord@ddb3ddb.

[vilgotf]

As an alternative to serde_json::Value, serde_json::value::RawValue could be used instead. Potentially hidden and only accessible through a str reference getter method.