Add end to end encryption

[yunkon-kim]

• Create sectuil package
• Add secret exchange and update mechanism
• Encrypt and decrypt messages between hosts (i.e., the cb-network agents)
• Test and debug the end-to-end encryption
• Do lint
• Add comments

[yunkon-kim]

[Reference/example]

• Generating RSA Private and Public Key Pair in Go Lang?
• 가장 빨리 만나는 Go 언어 Unit 53.3 RSA 공개키 알고리즘 사용하기
• jshap70/rsa_loading.go
• Base64 encoding and decoding in The Go Playground
• wongoo/rsautil.go
• RSA Private-Key format 과 PKCS#8
• Go cypto - 4. RSA 키 인코딩/디코딩
• 1. Map 개요 (for Keyring)