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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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"""Get a datetime.datetime or None from a string in the time format described in sshd_config(5)"""
    parsed_result = re.match(
        r"^(?P<prefix>[+-])((?P<weeks>\d+)[wW])?((?P<days>\d+)[dD])?((?P<hours>\d+)[hH])?((?P<minutes>\d+)[mM])?((?P<seconds>\d+)[sS]?)?$",
        relative_time_string)
    if parsed_result is None or len(relative_time_string) == 1:
        
        return None
    offset = datetime.timedelta(0)
    if parsed_result.group("weeks") is not None:
        offset += datetime.timedelta(weeks=int(parsed_result.group("weeks")))
    if parsed_result.group("days") is not None:
        offset += datetime.timedelta(days=int(parsed_result.group("days")))
    if parsed_result.group("hours") is not None:
        offset += datetime.timedelta(hours=int(parsed_result.group("hours")))
    if parsed_result.group("minutes") is not None:
        offset += datetime.timedelta(
            minutes=int(parsed_result.group("minutes")))
    if parsed_result.group("seconds") is not None:
        offset += datetime.timedelta(
            seconds=int(parsed_result.group("seconds")))
    if parsed_result.group("prefix") == "+":
        return datetime.datetime.utcnow() + offset
    else:
        return datetime.datetime.utcnow() - offset
def select_message_digest(digest_string):