Add environment variable placeholder support to VaultEnvironmentEncryptor

[wowapxah]

 ## Summary
 This PR adds support for environment variable placeholders in VaultEnvironmentEncryptor, allowing dynamic configuration of Vault paths using environment variables.
 
 ## Changes
 - Add `replaceEnvironmentPlaceholders` method to resolve `${VAR_NAME}` placeholders
 - Support environment variable substitution in vault keys and parameter names
 - Add comprehensive test coverage for various placeholder scenarios
 - Handle missing environment variables gracefully by keeping original placeholders
 - Add proper Javadoc documentation for new functionality
 
 ## Testing
 - All existing tests pass
 - Added 8 new test methods covering various scenarios
 - Tested with existing and non-existing environment variables
 
 ## Benefits
 This enhancement allows dynamic configuration of Vault paths using environment variables, making the configuration more flexible and environment-specific.