fix(rln-cli): improve configuration handling, add relay and stateless example

[vinhtc27]

Fix Configuration Handling:

• Document the expected format of the configuration file.
• Provide an example configuration file in rln-cli to guide users.
• Ensure that the CLI provides useful error messages if the configuration is missing or incorrect.

Implement Relay and Stateless RLN Examples

• Add Relay example to illustrate RLN’s practical use case.
• Add Stateless RLN example, where an externally built Merkle tree provides paths and roots for RLN stateless feature.