CLI fails to create COSE using Azure KeyVault RSA key

[ivarprudnikov]

Describe the bug

Cannot use sign operation with an RSA key. The underlying cose signing operation does not support RSA it seems https://github.com/microsoft/CCF/blob/a58d2c880b3691211ed7fa68b684e1ef5bfabf4c/python/ccf/cose.py#L82

To Reproduce

1. Generate a self signed RSA 4096 cert in AKV
2. Get the public key
   • az keyvault certificate download --vault-name $VAULTNAME -n $CERTNAME -f cabundle.pem -e PEM

3. Create AKV details json file for the CLI to use akv-config.json:

   {
     "keyVaultName":"$VAULTNAME","certificateName":"$CERTNAME","certificateVersion":"$version"
   }

4. Create a sample file to sign claim-foobar.txt

5. Invoke the CLI to create a signature:

   scitt sign --claims claim-foobar.txt --content-type "text/plain" --akv-configuration akv-config.json --x5c cabundle.pem --out claim.cose

6. The failure:

   Traceback (most recent call last):
     File "/home/ubuntu/.local/bin/scitt", line 11, in <module>
       load_entry_point('pyscitt', 'console_scripts', 'scitt')()
     File "/home/ubuntu/scitt-ccf-ledger/pyscitt/pyscitt/cli/main.py", line 42, in main
       args.func(args)
     File "/home/ubuntu/scitt-ccf-ledger/pyscitt/pyscitt/cli/sign_claims.py", line 211, in <lambda>
       func=lambda args: sign_claims(
     File "/home/ubuntu/scitt-ccf-ledger/pyscitt/pyscitt/cli/sign_claims.py", line 141, in sign_claims
       signed_claims = kv_client.cose_sign(
     File "/home/ubuntu/scitt-ccf-ledger/pyscitt/pyscitt/key_vault_sign_client.py", line 81, in cose_sign
       tbs = create_cose_sign1_prepare(
     File "/home/ubuntu/.local/lib/python3.8/site-packages/ccf/cose.py", line 143, in create_cose_sign1_prepare
       alg = default_algorithm_for_key(cert.public_key())
     File "/home/ubuntu/.local/lib/python3.8/site-packages/ccf/cose.py", line 96, in default_algorithm_for_key
       raise NotImplementedError("unsupported key type")
   NotImplementedError: unsupported key type

Expected behavior The command should not complain and sign the payload