Add mentions to trigger deployment failed notification

[khanhtc1202]

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

NONE

[ono-max]

Wow, nice idea 👍
/lgtm

[nakabonne]

Nice. Do we need to address the case of NotificationEventDeploymentRollingBack as well?

[nakabonne]

In either way, it is able to fall into another PR though.

[pipecd-bot]

Code coverage for golang is 31.91%. This pull request increases coverage by 0.03%.

File	Function	Base	Head	Diff
pkg/config/deployment.go	PostSync.Validate	--	66.67%	+66.67%
pkg/config/deployment.go	DeploymentChain.Validate	--	66.67%	+66.67%
pkg/config/deployment.go	DeploymentChainNode.Validate	--	75.00%	+75.00%
pkg/config/deployment.go	DeploymentChainTriggerCondition.Validate	--	0.00%	+0.00%
pkg/app/piped/trigger/trigger.go	Trigger.notifyDeploymentTriggerFailed	0.00%	0.00%	+0.00%
pkg/config/deployment.go	GenericDeploymentSpec.Validate	53.85%	56.25%	+2.40%

[khanhtc1202]

Nice. Do we need to address the case of NotificationEventDeploymentRollingBack as well?

Not yet, since we do not return that event anywhere. Should make a separated PR for that (include where we will notify that event to slack) cc @ono-max

[nakabonne]

Alright 👍
/approve

[pipecd-bot]

This pull request is APPROVED by nakabonne.

Approvers can cancel the approval by writing /approve cancel in a comment. Any additional commits also will change this pull request to be not-approved.