structerr: Extract metadata from joined errors

The structerr.ExtractMetadata() function extracts metadata from the specified error. If the error is wrapped, metadata for chained errors is also extracted. Wrapped errors implement Unwrap() error to allow chained erros to be uncovered.

As of Go version 1.20, errors can be joined with the errors.Join() function. Since joined errors internally reference multiple errors, to be unwrapped, joined errors implement Unwrap() []error instead.

To support extraction of error metadata when joined errors exist in the chain, structerr.ExtractMetadata() is updated to recursively unwrap standard errors and joined errors.