fix: Use correct JSON-LD context prefix

[leandro-cavalcante]

Description

In the returned policies, the policy hub adds the https://w3id.org/catenax/v0.0.1/ns/ context with the cx prefix, but then uses the cx-policy prefix instead.

Why

The cx-policy prefix isn’t bound to any context within the JSON-LD document and therefore will fail to expand during serialization.

Issue

N/A

Issue

Link to Github issue.

Checklist

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• I have followed the contributing guidelines
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• I have created and linked IP issues or requested their creation by a committer
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• I have successfully tested my changes locally
• I have added tests that prove my changes work
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[leandro-cavalcante]

@evegufy the PR #232 was closed due some issues I had within my branch. I opened this one and it already has the changes requested by @Phil91