* Support repeated Payload, add godoc comment regarding not supporting Payload directly

* Forgot to commit a change, remove slices import

_**READ BEFORE MERGING:** All PRs require approval by both Server AND
SDK teams before merging! This is why the number of required approvals
is "2" and not "1"--two reviewers from the same team is NOT sufficient.
If your PR is not approved by someone in BOTH teams, it may be summarily
reverted._

<!-- Describe what has changed in this PR -->
**What changed?**
Add "reason" to PauseActivityRequest

<!-- Tell your future self why have you made these changes -->
**Why?**
Product request.
Unlike other activity operation, pause is reversible. For manually
paused activities operator may need to know the reason for pausing.

<!-- Are there any breaking changes on binary or code level? -->
**Breaking changes**
No.

<!-- If this breaks the Server, please provide the Server PR to merge
right after this PR was merged. -->
**Server PR**

<!-- Describe what has changed in this PR -->
**What changed?**
Add "PauseInfo" message to the PendincgActivityInfo. Message contain
information about who, why and when paused the activity.
Note: if activity is paused by rule id, users will need to call
"DescribeWorkflowRule" to get the details for that rule.

<!-- Tell your future self why have you made these changes -->
**Why?**
Product request, for audit purpose. Note: we don't add this information
to the history.

<!-- Are there any breaking changes on binary or code level? -->
**Breaking changes**
No

…entAttributes (#573)

_**READ BEFORE MERGING:** All PRs require approval by both Server AND
SDK teams before merging! This is why the number of required approvals
is "2" and not "1"--two reviewers from the same team is NOT sufficient.
If your PR is not approved by someone in BOTH teams, it may be summarily
reverted._

<!-- Describe what has changed in this PR -->
**What changed?**
Clarify docs of `root_workflow_execution` in
`WorkflowExecutionStartedEventAttributes`

<!-- Tell your future self why have you made these changes -->
**Why?**

<!-- Are there any breaking changes on binary or code level? -->
**Breaking changes**

<!-- If this breaks the Server, please provide the Server PR to merge
right after this PR was merged. -->
**Server PR**

… (#572)

**What changed?**
Added ScheduledEventID of the Nexus operation to the new
NexusOperationCancelRequestCompleted and
NexusOperationCancelRequestFailed events

**Why?**
For efficient lookups. Go SDK stores cancellation state machines keyed
on scheduled event ID.

**Server PR**
temporalio/temporal#7612

<!-- Describe what has changed in this PR -->
**What changed?**
Added an ENUM `ApplicationErrorCategory`, used in
`ApplicationFailureInfo` proto.

<!-- Tell your future self why have you made these changes -->
**Why?**
Give users a way to categorize application failures. Error categories
map to different observability behaviours SDK-side (i.e.
logging/metrics), part of the work for benign exceptions. Categories
could also be used with rules in the future.

<!-- Are there any breaking changes on binary or code level? -->
**Breaking changes**
No

<!-- If this breaks the Server, please provide the Server PR to merge
right after this PR was merged. -->
**Server PR**
Don't believe this breaks server or requires any changes to server