…plementationOptions

Fixes temporalio#2790

Allow predefining ChildWorkflowOptions on WorkflowImplementationOptions,
mirroring the existing ActivityOptions, LocalActivityOptions and
NexusServiceOptions support:

- Add setChildWorkflowOptions(Map) and setDefaultChildWorkflowOptions()
  builder methods, matching getters, and equals/hashCode/toString/
  toBuilder support on WorkflowImplementationOptions
- Expose the options through SyncWorkflowContext, served directly from
  the immutable WorkflowImplementationOptions
- Add ChildWorkflowOptions.Builder#mergeChildWorkflowOptions(): non-null
  override fields win, except contextPropagators lists are concatenated
  (matching ActivityOptions.Builder#mergeActivityOptions), the mutually
  exclusive searchAttributes/typedSearchAttributes are merged as one
  logical field so the merged options never carry both flavors, and
  VERSIONING_INTENT_UNSPECIFIED is treated as unset
- Resolve the predefined options for both typed and untyped child stubs
  (WorkflowInternal.newChildWorkflowStub and newUntypedChildWorkflowStub,
  so DynamicWorkflow parents are covered too) with the following
  precedence, highest to lowest, merged field by field: options passed
  to the stub creation method > per-type options (setChildWorkflowOptions)
  > default options (setDefaultChildWorkflowOptions)
- Keep child stub creation cost unchanged: the interface metadata is
  computed once and passed into ChildWorkflowInvocationHandler instead
  of being reflectively re-derived
- Keep the previous public WorkflowImplementationOptions constructor as
  a backward-compatible delegating overload
- Document the new behavior on the Workflow child stub factory methods
  and warn against predefining workflowId, which would be applied to
  every child of the matching scope

Fix child workflow options precedence: the predefined options were
merged in the wrong order, so when no options were passed to
newChildWorkflowStub the default options overrode the per-type options.
The default options have the lowest precedence and must never override
per-type options. The merge order is now correct, and the javadocs on
setChildWorkflowOptions and setDefaultChildWorkflowOptions describe the
actual precedence.

Tests:
- Verify the applied options through the child's memo in
  DefaultChildWorkflowOptionsSetOnWorkflowTest (covering default,
  per-type, explicit and field-level merge precedence for both typed
  and untyped stubs), so a test only passes if the expected options
  actually took effect
- Add an exhaustive unit test for
  ChildWorkflowOptions#mergeChildWorkflowOptions that exercises every
  field, plus dedicated tests for the search attribute flavors, context
  propagator concatenation and unspecified versioning intent

Signed-off-by: Oleksandr Porunov <alexandr.porunov@gmail.com>