implement optional async support and remove nested result type in response

[wfraser]

The new features controlling this are async_routes which enables compiling the async functions, and and default_async_client which enables a default HTTP client based on reqwest.

For most current use cases this should be a fairly seamless transition, as I've made great pains to ensure that types are in the same locations unless you use the new features.

The one thing which will require manual updates is the return value from most functions has changed from Result<Result<T, E, dropbox_sdk::Error> to Result<T, dropbox_sdk::Error<E>>. That is, code which previously did:

match get_metadata(&client, &arg) {
    Ok(Ok(metadata)) => { ... }
    Ok(Err(api_error)) => { /* the request succeeded, but the API returned an error */ }
    Err(e) => { /* the request failed entirely */ }
}

should now look like:

match get_metadata(&client, &arg) {
    Ok(metadata) => { ... }
    Err(Error::Api(api_error)) => { /* the request succeeded, but the API returned an error */ }
    Err(e) => { /* the request failed entirely */ }
}

The thing is, most code doesn't look like this and just wants to use the ? operator to pass errors up, so that's now easier to do.

If users have implemented a custom HTTP client, they will need to rework it a lot, as the HTTP client traits have been completely rewritten and are very different. That said, the new trait moves more things to the helper code, so implementing a custom client should be simpler now.

This change violates our MSRV policy a bit (requiring rust 1.75.0 which is only 10 months old instead of the desired 12) so I'm proposing to land this as a beta release for now.

Checklist

General Contributing

• I have read the Code of Conduct and signed the CLA.
• I have added an entry to the RELEASE_NOTES.md file, or believe it's not necessary for this change.

Validation

A copy of the demo example has been made (called demo-async) which uses the default async client, which acts as a decent integration test.

The CI scripts have been updated to check things under both sync and async modes.

[wfraser]

As long as we're doing this big of a change, might as well do the error-flattening change too.