Enable decrypting the sealed secrets at piped

[nghialv]

What this PR does / why we need it:

In the next pull requests:

• more test for secrets management feature
• improve the visibility of the sealed secret

Which issue(s) this PR fixes:

Part of #749

Does this PR introduce a user-facing change?:

NONE

[nghialv]

/cc @nakabonne

[pipecd-bot]

Code coverage for golang is 26.22%. This pull request decreases coverage by -0.10%.

File	Function	Base	Head	Diff
pkg/app/piped/controller/controller.go	decryptSealedSecrets	--	0.00%	+0.00%
pkg/config/config.go	Config.GetGenericDeployment	--	0.00%	+0.00%
pkg/config/deployment.go	GenericDeploymentSpec.GetStage	--	0.00%	+0.00%
pkg/app/piped/controller/planner.go	planner.Run	0.00%	0.00%	+0.00%
pkg/app/piped/controller/scheduler.go	scheduler.ensurePreparing	0.00%	0.00%	+0.00%
pkg/config/config.go	Config.GetPipelineable	0.00%	--	+-0.00%
pkg/config/deployment_cloudrun.go	CloudRunDeploymentSpec.GetStage	0.00%	--	+-0.00%
pkg/config/deployment_kubernetes.go	KubernetesDeploymentSpec.GetStage	0.00%	--	+-0.00%
pkg/config/deployment_lambda.go	LambdaDeploymentSpec.GetStage	0.00%	--	+-0.00%
pkg/config/deployment_terraform.go	TerraformDeploymentSpec.GetStage	0.00%	--	+-0.00%

[nakabonne]

Except for just one nit, looks good to me. A big thank you goes out to you!

[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.

[pipecd-bot]

Code coverage for golang is 26.22%. This pull request decreases coverage by -0.10%.

File	Function	Base	Head	Diff
pkg/app/piped/controller/controller.go	decryptSealedSecrets	--	0.00%	+0.00%
pkg/config/config.go	Config.GetGenericDeployment	--	0.00%	+0.00%
pkg/config/deployment.go	GenericDeploymentSpec.GetStage	--	0.00%	+0.00%
pkg/app/piped/controller/scheduler.go	scheduler.ensurePreparing	0.00%	0.00%	+0.00%
pkg/app/piped/controller/planner.go	planner.Run	0.00%	0.00%	+0.00%
pkg/config/config.go	Config.GetPipelineable	0.00%	--	+-0.00%
pkg/config/deployment_cloudrun.go	CloudRunDeploymentSpec.GetStage	0.00%	--	+-0.00%
pkg/config/deployment_kubernetes.go	KubernetesDeploymentSpec.GetStage	0.00%	--	+-0.00%
pkg/config/deployment_lambda.go	LambdaDeploymentSpec.GetStage	0.00%	--	+-0.00%
pkg/config/deployment_terraform.go	TerraformDeploymentSpec.GetStage	0.00%	--	+-0.00%