Add DHI contribution license agreement

[ecomodeller]

One contribution license agreement for all DHI open-source repositories, so DHI can relicense them, including under a proprietary license, without asking each contributor.

The text is the Microsoft Contribution License Agreement (used for Code OSS), which Microsoft dedicates to the public domain under CC0. Only the party name (DHI A/S) and governing law (Denmark) are changed.

The reusable workflow runs CLA Assistant Lite and stores signatures centrally in DHI/cla-signatures, so a contributor signs once for all repos. A repo opts in with a short caller workflow, e.g. DHI/modelskill#723.

Before merge:

• DHI legal review of the text: it is a license, not a copyright assignment, and Danish governing law has not been reviewed.
• An org admin creates DHI/cla-signatures and an org secret CLA_SIGNATURES_TOKEN with write access to it.

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