Add Secret class

[RobertoPrevato]

• Add a Secret class to handle secrets in code instead of using plain str. This
  approach offers several advantages:

1. It encourages loading secrets from environment variables, and discourages programmers
   from hardcoding secrets in source code.
2. Avoids accidental exposure of secrets in logs or error messages, by overriding
   str and repr.
3. It causes exception if someone tries to JSON encode it using the built-in JSON
   module, unlike str.
4. For convenience, it can be compared directly to strings. It uses constant-time
   comparison to prevent timing attacks, with the built-in secrets.compare_digest.
5. Environment variables can be changed at runtime, using this class applications can
   pick up secret changes without needing to be restarted.

• Add an EnvironmentVariableNotFound exception that can be used when an expected env
  variable is not set.
• Improve the order of if statements in the FriendlyEncoder class to prioritize the
  most frequently encountered types first, which should provide better performance in
  typical use cases.