Add (draft) protobuf io.

[winterse26]

Added a draft version for graph io serializing with protobuf. Only a subset of vertex and edge types are covered. However, in our usecase we already got the benefits of less memory consumption and avoidance of precision loss due to the representation as string. The mapping between tags and protobuf enums, although, needs improvment.
If you think the addition could be valueable also for others, we could iterate towards a version that could be merged.

[RainerKuemmerle]

In general, I like the idea.
I'm just wondering a bit if we should reason about changes in the g2o API that make things a bit easier. For example, if we would ask each type to provide a toVector/fromVector function both serialization formats could be added on top. And we could drop the individual read/write functions in the types?
What do you think?