Refactor crypto package

[nghialv]

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

NONE

/cc @gkuga

[pipecd-bot]

Code coverage for golang is 26.23%. This pull request decreases coverage by -0.05%.

File	Function	Base	Head	Diff
pkg/crypto/aes.go	NewAESEncryptDecrypter	--	83.33%	+83.33%
pkg/crypto/aes.go	AESEncryptDecrypter.Encrypt	--	76.92%	+76.92%
pkg/crypto/aes.go	AESEncryptDecrypter.Decrypt	--	71.43%	+71.43%
pkg/crypto/decrypter.go	NewDecrypter	83.33%	--	-83.33%
pkg/crypto/decrypter.go	decrypter.Decrypt	71.43%	--	-71.43%
pkg/crypto/encrypter.go	NewEncrypter	83.33%	--	-83.33%
pkg/crypto/encrypter.go	encrypter.Encrypt	76.92%	--	-76.92%

[gkuga]

/approve

[pipecd-bot]

This pull request is APPROVED by gkuga.

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.