Replace server.GroupedError with some standard multierr package

[josephlr]

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Potential packages to use:

• https://pkg.go.dev/go.uber.org/multierr
• https://pkg.go.dev/github.com/hashicorp/go-multierror

This would make it easier for both us and our users to deal with functions that may return multiple errors.

Examples include:

• client.Attest
• server.VerifyAttestation