[crypto] Add AEAD class

This CL adds AEAD, a secret key cipher that can be used to encrypt and
decrypt data. AEADs are distinguished from Ciphers in that they do not
require block-aligned lengths and ensure data integrity, at the cost of
some additional space.

Change-Id: I51df60a004f55e763f18ec0d468a7a4ddb9f46f4

[uboringssl] Add crypto support for AEADs

This CL expands the cryptographic routines supported by the Zircon port
of BoringSSL to include those needed for AEADs.

Change-Id: I34fe0122dbb9b8070184c5d98f706b23cf3586d5