OpenBao support

[digiserg]

Implements comprehensive dual backend support allowing vals-operator to work with both
HashiCorp Vault and OpenBao. OpenBao is the open-source fork of Vault that maintains
API compatibility while being fully community-driven.

Key changes:
- Add abstraction layer with SecretsClient interface for backend-agnostic operations
- Implement OpenBao client wrapper using BAO_* environment variables
- Implement Vault client wrapper maintaining existing VAULT_* variables for backwards compatibility
- Add automatic environment variable fallback from BAO_* to VAULT_* variables
- Add workaround for vals library compatibility by setting VAULT_* variables when using OpenBao
- Update Helm chart to support both backends with proper configuration
- Update documentation with authentication configuration for both backends
- Fix initialization to check for either VAULT_ADDR or BAO_ADDR

Environment variable changes:
- OpenBao uses BAO_ prefix (BAO_ADDR, BAO_TOKEN, BAO_ROLE_ID, etc.)
- Falls back to VAULT_* variables when BAO_* not set for easy migration
- OpenBao takes precedence when both are configured

This provides zero-downtime migration path from Vault to OpenBao while maintaining
full backwards compatibility with existing deployments.