att init: create contract on workflow creation

[jiparis]

This PR allows to reuse --contract flag in att init to specify a contract file or URL to be used if the workflow has to be created. In this case, it will first create the contract with the file contents and pass the reference to the Workflow/Create call.

✗ chainloop att init --name qs-contract --project myproject --contract qs-contract.yaml --replace
INF Attestation initialized! now you can check its status or add materials to it
┌───────────────────────────┬──────────────────────────────────────┐
│ Initialized At            │ 13 Mar 25 19:27 UTC                  │
├───────────────────────────┼──────────────────────────────────────┤
│ Attestation ID            │ 5ba1b703-78f8-4e0f-825b-9cff3e4df7e8 │
│ Organization              │ my-org                               │
│ Name                      │ qs-contract                          │
│ Project                   │ myproject                            │
│ Version                   │ v0.181.0 (prerelease)                │
│ Contract                  │ myproject-qs-contract (revision 1)   │
│ Policy violation strategy │ ADVISORY                             │
└───────────────────────────┴──────────────────────────────────────┘

Fixes #1901