New Design for client -- broker protocol for Snowflake

This is related to the Snowflake protocol design tickets legacy/trac#29206 (moved) and legacy/trac#29207 (moved).

We want to write these protocols in a way that is not Snowflake-specific but allows the client to request any type of bridge from our broker/BridgeDB bridge distribution service.

Some things to keep in mind are that we'd like to give the clients multiple ways to connect to the bridge distributor and should get the necessary information for multiple types of bridges. We also need various ways for identifying or distinguishing clients that will aid our bridge partitioning system (e.g., salmon, hyphae). This could be by supplying email addresses, secrets, etc.