change the permission for the public key created by sda-cli to 644

[nanjiangshu]

Please describe the feature As a user, I want the default permission of the public key created by sda-cli to be all readable so that I know it can be shared with others. This is a feature requested by Joel during the sprint review on 2024-06-28

Currently the permission is 600 for both the public key and private key. Should be

-rw-r--r--   c4gh.pub.pem
-rw-------   c4gh.sec.pem 

Acceptance criteria

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Additional context

Estimation of size: small/medium/big

Estimation of priority: low/medium/high