Allow config values to be set from system environment values

[jjmark15]

Background

Currently all config values are defined in yaml config files. Therefore, all these values are in plaintext in version control. This would be inappropriate for sensitive values such as api secret keys which need to remain hidden.

Requirements

Add the ability to fill config values from specified environment variables.

TBD

• [x] What about fields that we only want to fill with environment vars some of the time?
  • possible solution: try to parse into EnvironmentReadValue, if fails then try to parse directly into T, if fails then bubble up failure

Describe the solution you'd like

yaml configs should dictate the name of the environment variable to read from in the format:

field: ${SECRET_ENV_VAR}

Then, in code we could have a type:

#[derive(Debug, serde::Deserialize)]
pub(crate) struct EnvironmentReadValue<T: serde::de::DeserializeOwned> {
    #[serde(flatten)]
    environment_variable_string: String,
    #[serde(skip)]
    value: Option<T>,
}

EnvironmentReadValue should be able to deserialize from a string such as "${SECRET_ENV_VAR}" and then return it's value of type T.

ToDos

• [x] resolve bubbling up errors reading values from environment when config values are accessed
  • could move to model where EnvironmentReadValue properties are used on a builder type. Then this builder type is converted into the target config type. The build process could be fallible but each access of the built type would be simple.
• [x] unit test env reading of web config port
• [x] remove derived serde traits from Config structs that don't need them

[jjmark15]

https://serde.rs/string-or-struct.html may be useful