Add Usage Type to SSH keys

What does this MR do and why?

Related issue: #375258 (closed)

Add an ability to create an SSH key of different usage types:

• For authentication only
• For signing only
• For both

Auth & Signing is the default value to make it easier for users start signing the commits: !97248 (comment 1172807823)

Test locally

In order to test locally, ssh_key_usage_types feature flag must be enabled:

rails c

=> Feature.enable(:ssh_key_usage_types)

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