Consolidate config_value and json_value

[Neverlord]

Currently, we have two separate ways of storing configuration values. For loading CLI and configuration file options, we have config_file. We also have an API for dealing with JSON. The latter uses a custom allocation strategy with a monotonic buffer resource. This allocator isn't thread-safe, which is why parsing a JSON document produces immutable values. Once produced, however, a json_value is cheap to copy around.

The actual implementation of the internal JSON values could be generalized and re-used. Either for new config formats (e.g., YAML or INI) or for using the same "backend" in config_value. However, this would most certainly introduce breaking API changes. So just filing as an idea for now.