fix: use Component.Variable[.Instance] key format for OCPP 2.0 variable persistence

fix: use Component.Variable[.Instance] key format for OCPP 2.0 variable persistence

The previous format used bare variable names (e.g., 'Enabled'), causing
collisions across components (9 components define Enabled, 8+ define
Available, etc.). Now uses the spec's (Component, Variable, Instance)
triplet as the persisted configuration key.

- Rewrite computeConfigurationKeyName to return Component.Variable format
- Remove shouldFlattenInstance (no longer needed)
- Extract buildConfigKey helper in ConfigurationKeyUtils, export via barrel
- Add MaxCertificateChainSize to OCPP20OptionalVariableName enum
- Update all direct config key lookups with component prefix
- Update keba-ocpp2 template keys to new format
- Update OCPP2_PARAMETER_KEY_MAP resolved values
- Replace all enum string literals in log messages with enum references
- Update all tests to use new key format