Add signatureValueFile fields for webhook notification

[knanao]

What this PR does / why we need it:
#3515

Which issue(s) this PR fixes:

Fixes #3515

Does this PR introduce a user-facing change?:

NONE

[pipecd-bot]

Code coverage for golang is 35.64%. This pull request increases coverage by 0.02%.

File	Function	Base	Head	Diff
pkg/config/piped.go	NotificationReceiverWebhook.LoadSignatureValue	--	80.00%	+80.00%
pkg/app/piped/notifier/webhook.go	webhook.sendEvent	0.00%	0.00%	+0.00%

[pipecd-bot]

Code coverage for golang is 35.64%. This pull request increases coverage by 0.02%.

File	Function	Base	Head	Diff
pkg/config/piped.go	NotificationReceiverWebhook.LoadSignatureValue	--	80.00%	+80.00%
pkg/app/piped/notifier/webhook.go	webhook.sendEvent	0.00%	0.00%	+0.00%

[nghialv]

Nice improvement.
/lgtm

[khanhtc1202]

🙌
/approve

[pipecd-bot]

This pull request is APPROVED by khanhtc1202.

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.