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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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self.privatekey = cryptography.hazmat.primitives.asymmetric.ec.generate_private_key(
                    curve=self.curves[self.curve]['create'](self.size),
                    backend=self.cryptography_backend
                )
        except cryptography.exceptions.UnsupportedAlgorithm as dummy:
            self.module.fail_json(msg='Cryptography backend does not support the algorithm required for {0}'.format(self.type))
        
        encryption_algorithm = cryptography.hazmat.primitives.serialization.NoEncryption()
        if self.cipher and self.passphrase:
            if self.cipher == 'auto':
                encryption_algorithm = cryptography.hazmat.primitives.serialization.BestAvailableEncryption(to_bytes(self.passphrase))
            else:
                self.module.fail_json(msg='Cryptography backend can only use "auto" for cipher option.')
        
        return self.privatekey.private_bytes(
            encoding=cryptography.hazmat.primitives.serialization.Encoding.PEM,
            format=export_format,
            encryption_algorithm=encryption_algorithm
        )
    def _load_privatekey(self):
        try:
            with open(self.path, 'rb') as f:
                return cryptography.hazmat.primitives.serialization.load_pem_private_key(
                    f.read(),
                    None if self.passphrase is None else to_bytes(self.passphrase),
                    backend=self.cryptography_backend
                )
        except Exception as e:
            raise PrivateKeyError(e)
    def _get_fingerprint(self):