Replace file_{[unique_]id,size} fields with FileMeta

[WaffleLapkin]

TL;DR, before:

pub struct Struct {
    /// An identifier for this file.
    pub file_id: String,

    /// Unique identifier for this file, which is supposed to be the same over
    /// time and for different bots. Can't be used to download or reuse the
    /// file.
    pub file_unique_id: String,

    /// File size in bytes.
    #[serde(default = "crate::types::file::file_size_fallback")]
    pub file_size: u32,

    // other fields...
}

after:

pub struct Struct {
    /// Metadata of the {media} file.
    #[deref]
    #[serde(flatten)]
    pub file: FileMeta,

    // other fields...
}

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I'm not certain that we should do this refactoring, but reusing FileMeta seems nice.

In some cases we can even use PhotoSize but I don't think it's worth it (also PhotoSize for videos would be confusing, I think).