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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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def __init__(self, path, state, force, check_mode):
        self.path = path
        self.state = state
        self.force = force
        self.name = os.path.basename(path)
        self.changed = False
        self.check_mode = check_mode
    def check(self, module, perms_required=True):
        """Ensure the resource is in its desired state."""
        def _check_state():
            return os.path.exists(self.path)
        def _check_perms(module):
            file_args = module.load_file_common_arguments(module.params)
            return not module.set_fs_attributes_if_different(file_args, False)
        if not perms_required:
            return _check_state()
        return _check_state() and _check_perms(module)
    @abc.abstractmethod
    def dump(self):