feat: add GCP Cloud KMS support for encryption/decryption

[gokulJuspay]

• Add GCPKMSClient for encrypting and decrypting data using GCP Cloud KMS
• Support authentication via GCP metadata server (OAuth2 tokens)
• Add GCPKMSConfig with builder pattern and from_env() for configuration
• Add encrypt() and decrypt() methods with base64 encoding support
• Enable json feature for reqwest to support API requests
• Add comprehensive unit tests for config builder and key resource paths

Environment variables:

• GCP_PROJECT_ID (required)
• GCP_KEY_RING (required)
• GCP_KEY_NAME (required)
• GCP_LOCATION (optional, default: global)
• GCP_KMS_BASE_URL (optional)
• GCP_METADATA_URL (optional)

[itz-Pratham]

u can make it pub fn build(self) -> Result<GcpKmsConfig, GcpConfigError>
in aws, since we have the crate, there is fallback. here there is none. So we should be strict here.

[sinha-sahil]

lets name it ckms