feat(#52): add set_registration_contract and player existence check in approve_milestone

[had3sgames]

(issue-52-approve-milestone-player-existence-check)

• Added registration_contract module via contractimport!
• Added set_registration_contract(env, addr) admin function
• approve_milestone: if DataKey::RegistrationContract is set, calls
  reg_client.try_get_player(&player_id) and returns PlayerNotFound on failure;
  skipped if not set
• Tests: skip path (no reg contract) and panic path (reg contract set, unknown
  player)

closes #52

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