PLT-8206 - Add GET endpoint to import contracts

[palas]

This PR adds an endpoint to the Marlowe Playground to import compressed JSON contracts from other apps (in particular from Marlowe Runner). The endpoint is called importContract and takes the compressed JSON of the contract as the parameter contract.

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• Branch
  • [ ] Tests are provided (if possible)
  • [x] Commit sequence broadly makes sense
  • [x] Key commits have useful messages
  • [x] Relevant tickets are mentioned in commit messages
  • [x] Formatting, materialized Nix files, PNG optimization, etc. are updated
• PR
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[paluh]

Really cool. Thanks a lot!