<!--- 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 -->

'1.2.840.113549.1.9.2': ('unstructuredName', ),
    '1.2.840.113549.1.9.3': ('contentType', ),
    '1.2.840.113549.1.9.4': ('messageDigest', ),
    '1.2.840.113549.1.9.5': ('signingTime', ),
    '1.2.840.113549.1.9.6': ('countersignature', ),
    '1.2.840.113549.1.9.7': ('challengePassword', ),
    '1.2.840.113549.1.9.8': ('unstructuredAddress', ),
    '1.2.840.113549.1.9.9': ('extendedCertificateAttributes', ),
    '1.2.840.113549.1.9.14': ('Extension Request', 'extReq'),
    '1.2.840.113549.1.9.15': ('S/MIME Capabilities', 'SMIME-CAPS'),
    '1.2.840.113549.1.9.16': ('S/MIME', 'SMIME'),
    '1.2.840.113549.1.9.16.0': ('id-smime-mod', ),
    '1.2.840.113549.1.9.16.0.1': ('id-smime-mod-cms', ),
    '1.2.840.113549.1.9.16.0.2': ('id-smime-mod-ess', ),
    '1.2.840.113549.1.9.16.0.3': ('id-smime-mod-oid', ),
    '1.2.840.113549.1.9.16.0.4': ('id-smime-mod-msg-v3', ),
    '1.2.840.113549.1.9.16.0.5': ('id-smime-mod-ets-eSignature-88', ),
    '1.2.840.113549.1.9.16.0.6': ('id-smime-mod-ets-eSignature-97', ),
    '1.2.840.113549.1.9.16.0.7': ('id-smime-mod-ets-eSigPolicy-88', ),
    '1.2.840.113549.1.9.16.0.8': ('id-smime-mod-ets-eSigPolicy-97', ),
    '1.2.840.113549.1.9.16.1': ('id-smime-ct', ),
    '1.2.840.113549.1.9.16.1.1': ('id-smime-ct-receipt', ),
    '1.2.840.113549.1.9.16.1.2': ('id-smime-ct-authData', ),
    '1.2.840.113549.1.9.16.1.3': ('id-smime-ct-publishCert', ),
    '1.2.840.113549.1.9.16.1.4': ('id-smime-ct-TSTInfo', ),
    '1.2.840.113549.1.9.16.1.5': ('id-smime-ct-TDTInfo', ),
    '1.2.840.113549.1.9.16.1.6': ('id-smime-ct-contentInfo', ),
    '1.2.840.113549.1.9.16.1.7': ('id-smime-ct-DVCSRequestData', ),
    '1.2.840.113549.1.9.16.1.8': ('id-smime-ct-DVCSResponseData', ),
    '1.2.840.113549.1.9.16.1.9': ('id-smime-ct-compressedData', ),