fix: Colon splitting for custom fields

[grace-rehn]

Update the custom fields parsing to split only on first :, allowing the value to contain :s.

Tested with:

- name: Create Release with Custom Fields
  uses: OctopusDeploy/create-release-action@grace/fix-custom-field-colon-splitting
  with:
    project: "Test GHA"
    custom_fields: |
      IMAGE_URL:123456789.dkr.ecr.us-east-1.amazonaws.com/service:latest
      BUILD_NUMBER:${{ github.run_number }}

Before

After

Fixes #638
Fixes FD-99

[hnrkndrssn]

LGTM 👍