Application limit for number of secrets in groups

Implement application limit for the maximum number of secrets that can be stored in a group 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 group, with caching for performance optimization.

Foundation services (SecretCountService, SecretsLimitEnforcement concern) were implemented in #585572.

Key Changes

• Limit enforcement: Integrate into GroupSecrets::CreateService to prevent exceeding limits
• Group-level configuration: Support for group-specific secret limits