Add private key field to metadata storage

This change adds the private key field to the metadata storage. It will be necessary for addressing #5 (closed).

PS: This change is backward compatible with the current stable version.

Note that this is only a part of the implementation of the new feature. Other MRs will be opened to address other necessary changes.

High level overview of the changes necessary for the new feature (for more details, refer to the issue):

• Prepare command create SSH public and private keys
• Prepare command persists the private key in the metadata storage and create Fargate Task that will use the public key in its container <- This MR implements the code to be used here
• Run command retrieve private key from metadata storage and uses it to connect to the container

Why was this MR needed?

This is a part of the solution for the issue #5 (closed).

Are there points in the code the reviewer needs to double check?

N/A

Does this MR meet the acceptance criteria?

• Documentation created/updated - not necessary for this moment
• Added tests for this feature/bug
• In case of conflicts with master - no conflicts

What are the relevant issue numbers?

#5 (closed)