Add WelcomeEncryption parameters to proto

[neekolas]

TL;DR

Added support for specifying post-quantum cryptography in MLS welcome messages.

What changed?

• Created a new encryption.proto file with a WelcomeEncryption enum that defines encryption types including post-quantum XWING_MLKEM_512
• Added the encryption ciphersuite field to welcome messages in the MLS API
• Updated the database intents to include encryption type information for installations

How to test?

1. Verify that the new WelcomeEncryption enum is properly defined with the expected values
2. Check that the welcome message structures correctly include the new ciphersuite field
3. Ensure that installation records in the database can properly store the encryption type

Why make this change?

This change prepares the MLS protocol implementation for post-quantum cryptography support, specifically adding the ability to specify MLKEM-512 as an encryption algorithm for welcome messages. This enhances security by providing quantum-resistant encryption options alongside traditional curve25519.

[neekolas]

• Add WelcomeEncryption parameters to proto #259 👈 (View in Graphite)
• main

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[macroscopeapp]

Add WelcomeEncryption parameters to MLS protocol messages to support CURVE25519 and XWING_MLKEM_512 encryption algorithms

• Adds new WelcomeEncryption enum in encryption.proto defining CURVE25519 and XWING_MLKEM_512 encryption types
• Extends WelcomeMessage.V1 and WelcomeMessageInput.V1 in mls.proto with ciphersuite field
• Adds encryption_type field to PostCommitAction.Installation in intents.proto

📍Where to Start

Start with the new enum definitions in encryption.proto which defines the core encryption types used by the other modified files.

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Macroscope summarized 516d0ca. (Automatic summaries will resume when PR exits draft mode or review begins).