[FEATURE] Improve handling of fields without a tag

[dragondgold]

Problem Statement

I load some configs from json/yaml files while secrets are loaded with secretfetch. The issue is I can't do this as secretfetch expects every field in the destination struct to have a valid tag, it returns the error "invalid tag for field Env: no secret tag found for field NonSecretValue"

This won't work

type Config struct {
        NonSecretValue string `yaml:"noSecretValue"`
        SecretValue string `secret:"aws=prefix/my-secret,env=SECRET_VALUE"`
}

The same happens with nested structs:

type Config struct {
        // This won't load as `API` struct does not have the `secret` tag
        API struct {
		AdminUser     string `secret:"aws=prefix/user,env=API_ADMIN_USER"`
		AdminPassword string `secret:"aws=prefix/pass,env=API_ADMIN_PASS"`
	}
}

Proposed Solution

It would be nice if I could just tell secretfetch to ignore fields without the secret tag (or make it the default behavior).

Alternative Solutions

To make this work right now I need to create another struct that has all the secrets and then assign them to my Config struct.

Example Usage

type Config struct {
        SecretValue string `secret:"aws=prefix/my-secret,env=SECRET_VALUE"`
	NonSecretValue string `yaml:"noSecretValue"`

        API struct {
		AdminUser     string `secret:"aws=prefix/user,env=API_ADMIN_USER"`
		AdminPassword string `secret:"aws=prefix/pass,env=API_ADMIN_PASS"`
	}
}
var cfg Config

// Read yaml config file
yaml.Unmarshal([]byte(data), &cfg)

// Read secrets, this will only load fields with the "secret" tag even if inside
// a nested struct without a tag. Other fields won't be changed.
secretfetch.Fetch(context.Background(), cfg, nil)

Benefits

A single and easy-to-use struct can be used to load non-secret values and secrets with secretfetch