Add protobuf encoding #42

[m-mohr]

Suggested by Bayer during the fiboa sprint: Add a protobuf encoding.

First thoughts:

• The field number assignment might be difficult due to the variable nature of fiboa and its extensions. But there are extensions and unknown fields
• The data types are limited, we need to check how to encode all the fiboa data types (but there are Any, OneOf and Maps)
• Check ProtoJSON - can we just send GeoJSON?
• The field names are a problem, we often use :, but that's not good in protobuf. Also, underscores have restrictions

Origin: fiboa/specification#42