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##### SUMMARY
Allow to specify a new "format" parameter in "openssl_privatekey". Today the output format is decided by a very simple heuristic which requires further commands to work properly.
I need, for example, to generate a RSA private key but in PKCS8 format. The current heuristic uses OpenSSH format for RSA keys and only uses PKCS8 for Ed25519 and similar. This makes me use a "command" with "openssl" to convert the generated key, which leads to idempotency problems and unnecessary complexity.

##### ISSUE TYPE
- Feature Idea

##### COMPONENT NAME
openssl_privatekey

##### ADDITIONAL INFORMATION
<!--- Describe how the feature would be used, why it is needed and what it would solve -->
The current heuristic should be kept unless overwritten by user.

<!--- Paste example playbooks or commands between quotes below -->
```yaml
openssl_privatekey:
  path: /etc/ssl/private/ansible.com.pem
  format: pkcs8
```
The values allowed are `pkcs1`, `raw` and `pkcs8`

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pass
    @abc.abstractmethod
    def _get_fingerprint(self):
        pass
    def generate(self, module):
        """Generate a keypair."""
        if not self.check(module, perms_required=False) or self.force:
            if self.backup:
                self.backup_file = module.backup_local(self.path)
            privatekey_data = self._generate_private_key_data()
            crypto_utils.write_file(module, privatekey_data, 0o600)
            self.changed = True
        self.fingerprint = self._get_fingerprint()
        file_args = module.load_file_common_arguments(module.params)
        if module.set_fs_attributes_if_different(file_args, False):
            self.changed = True
    def remove(self, module):