Drop use of TOML as an internal configuration value

[bruceg]

Thanks to its heritage as the former confirmation format of choice, Vector uses toml::Value (and toml::Table) in a number of places internally as an intermediate representation of the configs. Unfortunately, toml::Value can't represent None values, and so conversion from serde_json::Value into it can panic when such values are present. Vector should drop the use of TOML values internally in favor of the JSON Value type.

Related: #12832