TBD: Refine description of session_key encryption tunnel

[gijsvl]

An encryption tunnel was added that uses an AEAD schema using the session_key. The description in the spec is somewhat out of date and incomplete: https://github.com/boltlabs-inc/key-mgmt-spec/blob/develop/systems-architecture.md#opening-the-application-layer-authenticated-channel

Make sure to incorporate the following points:

• When transforming the session_key coming from the opaque protocol, we truncate it to use the first 32 bytes of the 64 provided bytes. Is this a good design or do we want to transform the 64 bytes into 32 bytes in another way?
• Associated data for encryption of messages is empty at this point. Do we want to add any meaningful associated data?