Add an 'extras' property to comments

[crazytonyli]

@oguzkocer I tried the "serde flatten" solution you mentioned in our 1:1, and it worked out great. One minor downside is that there will be additional parsing work required for all API endpoints. I tried to use RawValue to avoid that, but it does not seem to work with serde(flatten) (see serde-rs/json#1252).

WP.com endpoints may return extra information in the responses. We'll need to parse that information to support the app's functionalities. This PR introduces a mechanism to support that. We can add this block of code to the models if needed:

    #[serde(flatten)]
    #[WpContext(edit, embed, view)]
    pub extras: Option<JsonValue>,

[oguzkocer]

@crazytonyli I think this is a good start, but it looks like we have quite a few failing tests.

• We could consider marking the field with #[WpContextualOption] because there won't always be extra fields. I wonder if that'd also help with de-serializing speed.
• We might want to introduce a new type to hold the extras, instead of directly using JsonValue. This might be changed to be a uniffi::Object with some helpers in the future and having a type in advance could make things easier for us. It'd also look a bit more consistent when we use it in other types. Note that you probably need to use #[serde(transparent)] for this to work, but I don't know if #[serde(transparent)] & #[serde(flatten)] works together or not.
• The field ordering might matter, as I believe it does for de-serializing enums. That'd be a bit of an issue, because if we accidentally put this field above others, the ones below might always be set to None. If this is an issue, we could add a check in WpContext proc macro to ensure that if there is an extra field, it's at the end. Having a specific type would also help with this.
• If we have a separate type, we could consider moving the meta field to it. I am not so sure about this though, so just dropping it as an idea for now.

[crazytonyli]

The field ordering might matter

It does not seem so: https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=b62029454b6f50395fbb3dfd17a8d4b4.

BTW, I'll leave this PR open until we have an API endpoint that requires this change, like the WP.com /wp/v2/comments (see https://linear.app/a8c/issue/AINFRA-939).