[PB-5190] Add chat encryption

[TamaraFinogina]

Description

This PR adds chat key generation by the meeting moderator and its sharing with other meeting participants:

• Key is sent via hybrid channel (olm + Kyber)
• The moderator generates the key for the whole meeting and shares it with other participants
• Chat key is included in SAS
• Chat key is not changed upon the leaving/joining of participants (as encryption keys) because users must be able to read old messages as per the requirement

Related Issue

Similar PR for meet-web: meet-web

Motivation and Context

Chat messages are sent unencrypted in the original design; this PR changes that.

How Has This Been Tested?

Unit tests

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.