Make it easier to update multiple gist files

[mkrasnitski]

The UpdateGistBuilder API has a pain point if you want to edit multiple files and don't know the quantity of files to edit until runtime. As an example, if we have a Vec<(String, String)> containing pairs of filenames and contents to update the gist with, the best way I found to create a builder is the following:

let builder = crab.gists().update(id);

let data: Vec<(String, String)> = todo!();

let iter = data.into_iter();
let mut builder_with_files = None;
if let Some((file, content)) = iter.next() {
    builder_with_files = Some(builder.file(file).with_content(content));
}

while let Some((file, content)) = iter.next() {
    builder_with_files = Some(builder_with_files.unwrap().file(file).with_content(content));
}

// Manual .map() due to async-ness
let gist = match builder_with_files {
    Some(builder) => Some(builder.send().await?),
    None => None,
};

This is really ugly IMO and duplicates code more than it should. The problem here is that adding the first file is a type transformation of UpdateGistBuilder -> UpdateGistFileBuilder, whereas adding any subsequent files operates only on a UpdateGistFileBuilder. One way to fix this would require adding a way to obtain the original UpdateGistBuilder after adding a file, potentially by making UpdateGistFileBuilder::build public. However, there may also be other valid approaches.