Add AWS Secrets Manager/KMS node key support

[YoshihitoAso]

This pull request introduces support for securely managing P2P node keys using AWS Secrets Manager and optional AWS KMS decryption, allowing node keys to be fetched and decrypted at runtime rather than stored directly on disk. The changes include new CLI flags, TOML configuration options, and the implementation of fetcher and decrypter logic for AWS-based node key management. Documentation and tests are also added to ensure correct usage and behavior.

AWS Node Key Management Integration

• Added support for fetching node keys from AWS Secrets Manager and decrypting them with AWS KMS, including new fetcher (NodeKeyAwsSecretsManagerFetcher) and decrypter (NodeKeyAwsKmsDecrypter) implementations. [1] [2]
• Introduced a Manager class to coordinate node key retrieval and decryption based on CLI and TOML configuration, supporting both plaintext and encrypted flows.
• Created AwsClient utility for initializing AWS SDK clients and validating configuration.

CLI and Configuration Enhancements

• Added new CLI flags --nodekeysource and --nodekeydecryption to select node key source and decryption scheme, defaulting to file/plaintext but supporting AWS flows. [1] [2]
• Extended TOML configuration to allow specifying AWS secret name, version, KMS key ID, and encryption algorithm. [1] [2]
• Updated setNodeKey logic in flags.go to handle new source/decryption flows, with legacy flags taking precedence.

Documentation

• Added docs/aws-nodekey.md and updated README.md to describe AWS node key management, CLI options, configuration, and IAM requirements.