Introduce SDK flag to use user DC on Memo's

[yuandrew]

What was changed

Introduced an SDK flag, SDKFlagMemoUserDCEncode, to configure new behavior, where we actually use the user data converter when encoding Memo's. If user DC errors out, the default DC will be used as a fallback. If that also returns an error, the user DC error will be returned to the caller.

SDK flag is still set to false by default, so no behavior changes. After a Go SDK release or two, will the flag then be enabled by default.

Why?

Correct behavior to match other SDKs

Checklist

1. Closes Memo does not go through user provided data coverter #1045

2. How was this tested:
   Added new tests that flip on/off the SDK flag and validates behavior

3. Any docs updates needed?

---

Note

Adds SDKFlagMemoUserDCEncode to optionally encode workflow/schedule memos with the user data converter (falling back to default), and updates codepaths and tests to honor it.

• Flags & config:
  • Introduce SDKFlagMemoUserDCEncode with env toggle MEMO_USER_DC_ENCODE and setter SetMemoUserDCEncode.
• Memo encoding behavior:
  • Add encodeMemoValue and shouldUseMemoUserDataConverter; prefer user DataConverter for memo encoding, fallback to default on error.
  • Extend getWorkflowMemo(..) and validateAndSerializeMemo(..) to accept a memoFlagAccessor and apply flag.
  • Update all call sites (StartWorkflow, SignalWithStart, child workflow start, schedules, testsuite) to pass accessor and/or use new helpers.
• Schedule client:
  • encodeScheduleWorkflowMemo now uses user DC (with fallback) and defaults DC when nil.
• Tests:
  • Add tests validating memo encoding with old/new behavior and failure paths for both workflow and schedule clients.
  • Adjust testsuite memo paths to use new accessor; minor assertions tightened.

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[Quinn-With-Two-Ns]

The comment says this is useful for tests, but I don't see this used in the SDK anywhere. Why did we need to add this?

[yuandrew]

It's set and used in SetMemoUserDCEncode(), so we can defer setting it back to its previous flag