Add attach and mask functions for joining errors in a different way

[jsternberg]

The Attach and Mask functions create a new error node that returns the error message from one node, but creates a structure where the additional errors are in the error chain and can be discovered with errors.Is and errors.As.

[jsternberg]

There's two usages I envision with the above functions.

1. An alternative to the current stack trace implementation. Stack can be built off of Attach instead of collapsible error and I think it would be easier to understand.
2. A way of integrating errdefs with applications that don't use errdefs natively. You can take an existing error and use errdefs.Mask(err, errdefs.ErrInvalidArgument) and that will add invalid argument to the error chain so errgrpc works appropriately, but won't change the underlying message. This isn't possible with errors.Join or fmt.Errorf.

[dmcgowan]

If we add these functions in, we should make sure use of them in stack and errgrpc package to ensure the interface aligns with what we need there.

[AkihiroSuda]

ping @jsternberg