Refactor + Minor Fixes in key gen and key import commands

[sbs2001]

Refactors:

• [X] Use the ConfirmPrompt function everywhere instead of rolling prompt implementation everywhere.
• [ ] Move pkg/cosign/common.go to internal package.
• [ ] Maybe have a single function for writing + handling perms for private & public key.

Fixes:

Address https://github.com/sigstore/cosign/blob/1d224d924197d3fab3f624a5cbe27f25d934e9f2/cmd/cosign/cli/generate/generate_key_pair.go#L101 and https://github.com/sigstore/cosign/blob/1d224d924197d3fab3f624a5cbe27f25d934e9f2/cmd/cosign/cli/importkeypair/import_key_pair.go#L52

[sbs2001]

@asraa and @venafi-iw could you please clarify what the TODO precisely means ;) Is it perm locking down in the sense of doing something equivalent to chattr +i <key_file> ?

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