Expose raw Failure and Payload protos on SDK error and result types

[brucearctor]

Objective

Fixes #2203 by exposing the full underlying proto data for errors and results from update/activities.

SDK users generally expect to be able to extract all raw details about failures and payloads when high-level helper methods do not cover all use cases (e.g., retrieving stack traces from the CLI without parsing error strings, or getting the original payload representation without relying on a data converter).

Changes

• Error types: Renamed failure() to Failure() on the internal temporalError type, which is embedded in other public error types such as ApplicationError, TimeoutError, and ActivityError. This allows SDK users to retrieve the underlying *failurepb.Failure object directly. Adjusted the internal FailureHolder back to failureHolder to avoid method/field conflicts on NexusOperationError.
• Result types: Added an optional ValuesPayloads interface to converter and a GetPayloads helper.
  • Why not add Payloads() *commonpb.Payloads directly to the EncodedValue and EncodedValues interfaces? Doing so would be a backwards-incompatible breaking change for any users outside of sdk-go that have implemented or mocked these interfaces.
  • With GetPayloads(encoded interface{}) *commonpb.Payloads, existing implementations won't break, but Temporal's internal EncodedValue implementations now satisfy the new interface, allowing extraction of the raw *commonpb.Payloads.
• Updated the generated testing mocks.