Improving the certificate download procedure

[eunyoung14]

To issue a certificate from the Alpacon CLI, you need to:

1. Submit a CSR,
2. Find the certificate ID corresponding to the CSR, and
3. Download the certificate using the certificate ID.

However, finding the certificate ID corresponding to the CSR can be cumbersome. It would be helpful to simplify this process. Ideally, the result of submitting a CSR should include a command to download the corresponding certificate in one step.

alpacon csr create
...
...
You can obtain the certificate via the following command:
alpacon csr download-crt <csr_id>

The csr_id can be found in the API response.