📦 new: use configbuilder to parse config files into `Config` struct

[gaukas]

We add configbuilder package which provides support on parsing JSON and Protocol Buffer formatted config file into Config struct, enabling distributing config files in serializable formats.

[gaukas]

Re-requesting review for an update to a deprecated field. No functional change but the comments.

We would discourage the use of argv and env in terms of configuring the WATM in common use cases. However, since it is possible to find a more legitimate use of these two, we will avoid adding // Deprecate: flag to reduce unnecessary warning.