[WaitApproval Plugin] Add NotifyEvent methods

[t-kikuc]

What this PR does:

I implemented NotifyEvent methods in both piped and plugin side.

Why we need it:

The WaitApproval plugin needs to:

• notify approvers when the stage starts (NotificationEventDeploymentWaitApproval)
• notify when approval is completed (NotificationEventDeploymentApproved)

cf.

pipecd/pkg/app/piped/executor/waitapproval/waitapproval.go

Lines 112 to 143 in 2ce020e

	func (e *Executor) reportApproved(approver string) {
	users, groups, err := e.getApplicationNotificationMentions(model.NotificationEventType_EVENT_DEPLOYMENT_APPROVED)
	if err != nil {
	e.Logger.Error("failed to get the list of users or groups", zap.Error(err))
	}
	e.Notifier.Notify(model.NotificationEvent{
	Type: model.NotificationEventType_EVENT_DEPLOYMENT_APPROVED,
	Metadata: &model.NotificationEventDeploymentApproved{
	Deployment: e.Deployment,
	Approver: approver,
	MentionedAccounts: users,
	MentionedGroups: groups,
	},
	})
	}
	func (e *Executor) reportRequiringApproval() {
	users, groups, err := e.getApplicationNotificationMentions(model.NotificationEventType_EVENT_DEPLOYMENT_WAIT_APPROVAL)
	if err != nil {
	e.Logger.Error("failed to get the list of users or groups", zap.Error(err))
	}
	e.Notifier.Notify(model.NotificationEvent{
	Type: model.NotificationEventType_EVENT_DEPLOYMENT_WAIT_APPROVAL,
	Metadata: &model.NotificationEventDeploymentWaitApproval{
	Deployment: e.Deployment,
	MentionedAccounts: users,
	MentionedGroups: groups,
	},
	})
	}

Which issue(s) this PR fixes:

Part of #5367

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[t-kikuc]

I referred to here:

pipecd/pkg/app/piped/executor/waitapproval/waitapproval.go

Lines 145 to 158 in 93e9a27

	// getMentionedUsers returns the list of users groups who should be mentioned in the notification.
	func (e *Executor) getApplicationNotificationMentions(event model.NotificationEventType) ([]string, []string, error) {
	n, ok := e.MetadataStore.Shared().Get(model.MetadataKeyDeploymentNotification)
	if !ok {
	return []string{}, []string{}, nil
	}
	var notification config.DeploymentNotification
	if err := json.Unmarshal([]byte(n), &notification); err != nil {
	return nil, nil, fmt.Errorf("could not extract mentions users and groups config: %w", err)
	}
	return notification.FindSlackUsers(event), notification.FindSlackGroups(event), nil
	}

[codecov]

Codecov Report

Attention: Patch coverage is 4.83871% with 59 lines in your changes missing coverage. Please review.

Project coverage is 26.52%. Comparing base (93e9a27) to head (49402ed).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/app/pipedv1/cmd/piped/grpcapi/plugin_api.go	0.00%	47 Missing ⚠️
pkg/plugin/sdk/client.go	0.00%	11 Missing ⚠️
pkg/app/pipedv1/cmd/piped/piped.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5613      +/-   ##
==========================================
- Coverage   26.58%   26.52%   -0.06%     
==========================================
  Files         474      475       +1     
  Lines       50546    50660     +114     
==========================================
+ Hits        13438    13439       +1     
- Misses      36045    36158     +113     
  Partials     1063     1063              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[Warashi]

Almost LGTM. I commented on one nitpick.

[Warashi]

[nits]
We can use nil as the zero-value of slices.

Suggested change

	if err != nil {
	return []string{}, []string{}, err
	}
	if !n.Found {
	return []string{}, []string{}, nil
	}
	if err != nil {
	return nil, nil, err
	}
	if !n.Found {
	return nil, nil, nil
	}

[t-kikuc]

@Warashi Thank you! I fixed it here 49402ed

[Warashi]

LGTM