Generate keys into the temp folder rather than the current directory

[eritbh]

Also probably split the key creation logic into its own function in common.sh and just keep the high-level logic in op-create-identity

1password-ssh-utils/op-create-identity

Lines 101 to 105 in 9e828e9

	# TODO: generate this somewhere it won't be written to disk (use temp dir?)
	# TODO: can we read the new keys into variables here and immediately delete
	# the file to avoid having to set another variable to remember to
	# delete it later?
	echo "Generating new keypair..."