This change adds utility methods to generate packets incrementally. It
supports UDP, ESP, IPv4, IPv6 packet generation.
For ESP, it exclusively does AES-CBC, HMAC-SHA256.
Bug: 72950854
Test: This
Change-Id: Icffeed2ebb2005d79faf04f48fd5126d1d6fb175
Merged-In: Icffeed2ebb2005d79faf04f48fd5126d1d6fb175
(cherry picked from commit 0e4743d56553d698ac45ae548f31019ea6e91541)