Add `Einspeisung` BO Object and Enums (`EEGVermarktungsform`, `FernsteuerbarkeitStatus`)

[hamidhajiparvaneh]

Overview:
This task is about adding the Einspeisung Business Object (BO) to the project along with the associated enums. The changes needed are outlined in the following two pull requests:
PR 459
PR 460

Changes Required:

• Add Einspeisung BO:
  • Add the Einspeisung object, including its properties as defined in PRs
• Define Enums:
  • Add EEGVermarktungsform enum as defined in PRs
  • Add FernsteuerbarkeitStatus enum as defined in PRs

Acceptance Criteria:
The Einspeisung BO and the enums are added as per the changes in PRs 459 and 460.
Unit tests are updated or created to ensure the new BO and enums work as expected.