Customize deserializer in plugins so can use InvalidConfigValue when serde_json parsing fails.

[j-lanson]

Right now, in some plugins we have a #[derive(Deserialize)] on a struct Config to auto-parse JSON-value from set_config() into a struct. However, this doesn't give us information as to what field caused a failure if one occurs. Update the code to return InvalidConfigValue where appropriate. You can test this by changing the type of an int config value in a policy file to a float.