Write Protobufs for any BridgeDB data which must be sent over a network or IPC channel

BridgeDB should have Protobufs for any data structures which must be sent over either network or IPC channels. This includes data such as bridges parsed from Stem (which should be sent to the database manager from legacy/trac#12031 (moved)), any data which is going to be exported to CollecTor (e.g. if we were to redesign a new pool "assignments.log" format like for legacy/trac#2755 (moved) and exported that), and any data which the client-side Social Distributor (legacy/trac#7520 (moved)) built into a Tor Browser extension plans to send to BridgeDB and vice-versa.

Protocol buffers have had extensive security reviews, are used extensively in many projects, and would provide automatic code generation for serialisers/marshallers for Python/Java/C++/C/Go, meaning that, for example, both Metrics and BridgeDB could use the same generated code to read the same data format.