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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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result = dict()
    backend = cert._backend
    x509_obj = cert._x509
    for i in range(backend._lib.X509_get_ext_count(x509_obj)):
        ext = backend._lib.X509_get_ext(x509_obj, i)
        if ext == backend._ffi.NULL:
            continue
        crit = backend._lib.X509_EXTENSION_get_critical(ext)
        data = backend._lib.X509_EXTENSION_get_data(ext)
        backend.openssl_assert(data != backend._ffi.NULL)
        der = backend._ffi.buffer(data.data, data.length)[:]
        entry = dict(
            critical=(crit == 1),
            value=base64.b64encode(der),
        )
        oid = _obj2txt(backend._lib, backend._ffi, backend._lib.X509_EXTENSION_get_object(ext))
        result[oid] = entry
    return result