<!--- Verify first that your feature was not already discussed on GitHub -->
<!--- Complete *all* sections as described, this form is processed automatically -->

##### 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`

<!--- HINT: You can also paste gist.github.com links for larger files -->

module = AnsibleModule(
        argument_spec=dict(
            state=dict(type='str', default='present', choices=['present', 'absent']),
            size=dict(type='int', default=4096),
            type=dict(type='str', default='RSA', choices=[
                'DSA', 'ECC', 'Ed25519', 'Ed448', 'RSA', 'X25519', 'X448'
            ]),
            curve=dict(type='str', choices=[
                'secp384r1', 'secp521r1', 'secp224r1', 'secp192r1', 'secp256r1',
                'secp256k1', 'brainpoolP256r1', 'brainpoolP384r1', 'brainpoolP512r1',
                'sect571k1', 'sect409k1', 'sect283k1', 'sect233k1', 'sect163k1',
                'sect571r1', 'sect409r1', 'sect283r1', 'sect233r1', 'sect163r2',
            ]),
            force=dict(type='bool', default=False),
            path=dict(type='path', required=True),
            passphrase=dict(type='str', no_log=True),
            cipher=dict(type='str'),
            backup=dict(type='bool', default=False),
            select_crypto_backend=dict(type='str', choices=['auto', 'pyopenssl', 'cryptography'], default='auto'),
        ),
        supports_check_mode=True,
        add_file_common_args=True,
        required_together=[
            ['cipher', 'passphrase']
        ],
        required_if=[
            ['type', 'ECC', ['curve']],
        ],
    )
    base_dir = os.path.dirname(module.params['path']) or '.'