[ENHANCEMENT] Add an option to load values from different source

[vnenkpet]

Something like this:

@ValueFrom([
    new SecretFromFileCustomGetter().getValue('JWT_SECRET'), // priority
    new EnvValueGetter().getValue('JWT_SECRET'), // fallback
])
readonly jwtSecret: string;

where SecretFromFileCustomGetter, EnvValueGetter would be classes implementing interface like this:

export interface ValueGetter {
   getValue(key: string): string;
}

and this:

@Env('JWT_SECRET')
readonly jwtSecret: string;

would then simply be a shorthand for this:

@ValueFrom([
    new EnvValueGetter('JWT_SECRET'),
])
readonly jwtSecret: string;

(@Env, @ValueFrom, @EnvValueGetter and ValueGetter interface would all be provided by this library, and devs can implement their own value getter, like SecretFromFileCustomGetter).

The purpose for this is for people to be able to read config values from different sources than environment variables or .env file.

They should then be able to write their own shorthand for simplicity and this should be documented, like:

@SecretFromFile('JWT_SECRET')
readonly jwtSecret: string;

[ondra-m]

What if I need value from 3 sources?

For example DB password

1. For PRD it will be stored as secret
2. For DEV it will be stored as env
3. For local it will be stored as default

Something like this?

@Secret("DB_PASSWORD")
@Env("DB_PASSWORD")
@Default("postgres")
@IsString()
readonly dbPassword: string

I thinking that decorators is too "heavy" for such as simple task. What about this?

class Config extends Configuration {
  readonly dbPassword = value({
    fromSecret: "DB_PASSWORD",
    fromEnv: "DB_PASSWORD",
    default: () => "default",
    type: "string",
    trim: true,
    parseAs: "password",
    exposeInDevConsole: false,
  })
}