Fix GetCertificateStatusResponse.status type to Certificate15118EVStatus instead of genericStatus - ocpp2.0.1

[dwibudut]

Proposed changes

• Fix type of field GetCertificateStatusResponse.Status to types.Certificate15118EVStatus instead of types.GenericStatus
• Add omitempty to certificateType field base on doc. spec requirement
• Also fix panic nil pointer confirmation.StatusInfo.ReasonCode in unit test TestGetCompositeScheduleE2EMocked

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Further comments

Fix type of field GetCertificateStatusResponse.Status to types.Certificate15118EVStatus instead of types.GenericStatus:

Add omitempty to certificateType field on GetInstalledCertificateIdsRequest base on doc. spec: