Application limit for number of secrets in projects

Implement application limit for the maximum number of secrets that can be stored in a project within GitLab Secrets Manager.

Implementation plan

Implement secret count limits for GitLab Secrets Manager as part of GA readiness. The implementation enforces configurable limits on the number of secrets that can be stored per project and group, with caching for performance optimization.

Key Changes

• SecretCountService: Retrieves secret counts from OpenBao via LIST API
• SecretsLimitEnforcement concern: Reusable logic for limit validation across services
• Limit enforcement: Integrated into ProjectSecrets::CreateService to prevent exceeding limits