<!--- 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 -->

'1.2.840.10045.1.2': ('characteristic-two-field', ),
    '1.2.840.10045.1.2.3': ('id-characteristic-two-basis', ),
    '1.2.840.10045.1.2.3.1': ('onBasis', ),
    '1.2.840.10045.1.2.3.2': ('tpBasis', ),
    '1.2.840.10045.1.2.3.3': ('ppBasis', ),
    '1.2.840.10045.2': ('id-publicKeyType', ),
    '1.2.840.10045.2.1': ('id-ecPublicKey', ),
    '1.2.840.10045.3': ('ellipticCurve', ),
    '1.2.840.10045.3.0': ('c-TwoCurve', ),
    '1.2.840.10045.3.0.1': ('c2pnb163v1', ),
    '1.2.840.10045.3.0.2': ('c2pnb163v2', ),
    '1.2.840.10045.3.0.3': ('c2pnb163v3', ),
    '1.2.840.10045.3.0.4': ('c2pnb176v1', ),
    '1.2.840.10045.3.0.5': ('c2tnb191v1', ),
    '1.2.840.10045.3.0.6': ('c2tnb191v2', ),
    '1.2.840.10045.3.0.7': ('c2tnb191v3', ),
    '1.2.840.10045.3.0.8': ('c2onb191v4', ),
    '1.2.840.10045.3.0.9': ('c2onb191v5', ),
    '1.2.840.10045.3.0.10': ('c2pnb208w1', ),
    '1.2.840.10045.3.0.11': ('c2tnb239v1', ),
    '1.2.840.10045.3.0.12': ('c2tnb239v2', ),
    '1.2.840.10045.3.0.13': ('c2tnb239v3', ),
    '1.2.840.10045.3.0.14': ('c2onb239v4', ),
    '1.2.840.10045.3.0.15': ('c2onb239v5', ),
    '1.2.840.10045.3.0.16': ('c2pnb272w1', ),
    '1.2.840.10045.3.0.17': ('c2pnb304w1', ),
    '1.2.840.10045.3.0.18': ('c2tnb359v1', ),
    '1.2.840.10045.3.0.19': ('c2pnb368w1', ),
    '1.2.840.10045.3.0.20': ('c2tnb431r1', ),
    '1.2.840.10045.3.1': ('primeCurve', ),