Make all error types Send + Sync

[atro7318]

I am trying to use this library for calling the Zitadel API in a multi-threaded environment, thus I need all error types to be Send + Sync. Making errors Send + Sync is also recommended as a Rust API best practice. See the second through fourth paragraphs from this section:

Additionally, error types should implement the Send and Sync traits. An error that is not Send cannot be returned by a thread run with thread::spawn. An error that is not Sync cannot be passed across threads using an Arc. These are common requirements for basic error handling in a multithreaded application.

Send and Sync are also important for being able to package a custom error into an IO error using std::io::Error::new, which requires a trait bound of Error + Send + Sync.

One place to be vigilant about this guideline is in functions that return Error trait objects, for example reqwest::Error::get_ref. Typically Error + Send + Sync + 'static will be the most useful for callers. The addition of 'static allows the trait object to be used with Error::downcast_ref.

I first tried to use the currently used custom_error dependency, but it seems to only support Box<dyn Error>, not Box<dyn Error + Send + Sync + 'static>. I instead switched all of the error type definitions in this repo to be annotated with thiserror derive and attribute macros. I then changed all usages of Box<dyn Error> to Box<dyn Error + Send + Sync + 'static> and got rid of the custom_error dependency. I believe this solution minimizes boilerplate just as well as custom_error, allows for writing more code in plain Rust (rather than function-like macros), and allows for this repo's dependency on a much more widely adopted and maintained crate (thiserror).

If you agree, merging and releasing this change would be very useful for my work. Thank you.