chore: add json tags to mta_ext schema

[Haegi]

Description

This PR adds json tags to the MTA Ext schema, which are already there for the MTA schema.
This enables users to also use the kubernetes yaml parser, which is working on json tags.

Checklist

• Code compiles correctly
• Relevant tests were added (unit / contract / integration)
• Relevant logs were added
• Formating and linting run locally successfully
• All tests passing
• UA review
• Design is documented
• Extended the README / documentation, if necessary
• Open source is approved

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