Custom X509CertificateDataExtensions

[Bishop19]

This PR is inspired by ISO 18013-5, in which I need to define some attributes and set them on the extensions of a certificate. Therefore, this PR:

• adds support for custom extensions
  • a new param on generateEccCsrPem to represent the custom extensions that a user wants to add to the CSR
    • the value of an extension can be String, int, List<String> or bool
    • I tried to maintain compatibility with the existing SAN extension
• adds parsing for new extensions on X509CertificateDataExtensions (subjectKeyIdentifier, authorityKeyIdentifier and authorityInfoAccess)
  • if it is a known extension and there is no parsing for it, it just prints that (change this? Maybe add List<String> unsupportedExtensions field, because throwing an Exception would block all parsing)

TODO:

• add support for custom extensions on generateRsaCsrPem (if generateEccCsrPem looks good)

Note:

• I couldn't get build_runner to run and change the *.g.dart files. Only if I set the minimum sdk version to 3.0.0. Since I don't know if you want that change, I modified these files by hand (despite the warning). If you want that, I can just commit that change
• Would help with How to request extensions in generateRsaCsrPem #74 ?

[Bishop19]

@Ephenodrom bump 🙏🏼