feature: implemented @current and @next iteration selection

[leonsteinhaeuser]

This PR implements the desired functionality from the discussion: #42

In order to use this functionality, simply pass @current or @next as the iteration value.

JSON:

[
    {
        "name": "Iteration",
        "type": "iteration",
        "value": "@current"
    }
]

---

Example workflow:

name: Project automations (organization)

on:
  issues:
  pull_request:

env:
  gh_project_token: ${{ secrets.PAC_TOKEN }}
  project_id: 1
  gh_organization: sample-org
  custom_field_values: '[{\"name\": \"Iteration\",\"type\": \"iteration\",\"value\": \"@current\"}]'

jobs:
  issue_project_custom_field_update:
    name: issue_opened_or_reopened
    runs-on: ubuntu-latest
    if: github.event_name == 'issues'
    needs:
      - issue_opened_or_reopened
    steps:
      - name: 'Modify custom fields'
        uses: leonsteinhaeuser/project-beta-automations@v1.3.0-alpha.1
        env:
          DEBUG_LOG: "true"
        with:
          gh_token: ${{ env.gh_project_token }}
          organization: ${{ env.gh_organization }}
          project_id: ${{ env.project_id }}
          resource_node_id: ${{ github.event.issue.node_id }}
          operation_mode: custom_field
          custom_field_values: ${{ env.custom_field_values }}