Added support for the Key Usage Exension of X.509 certificates

[Nikoo00o]

Hi, i used the package to create self signed certificates, but i also needed the key usage extension. Before the package only supported the extended key usage extension, so i added the support for the default key usage extension and created this pull request so that it could maybe be included in the next release of the package.

For reference: https://www.openssl.org/docs/manmaster/man5/x509v3_config.html#Key-Usage

[Ephenodrom]

@Nikoo00o Thank you for the work, this looks really good! Will merge and update on pub.dev next week at the latest