Extend functionality of CertificateExtensions

[afk11]

Adds some useful methods to CertificateExtensions, namely those for a certificate authority (subject key identifier, issuer key identifier, basic constraints, and key usage.

Also adds a KeyUsage static class, to produce bitmap of the allowed use of the certificate by clients.

[fgrosse]

Hey thanks for yet another contribution afk11

I will review this today but I can already see that the build on php failed due to:

FG\Test\X509\CertificateExtensionTest::testCaExtensions
Only variables should be passed by reference

Also I fixed the style CI settings and consider the issues that come up there to be actual code style issues that need to be resolved before I can merge this.

[fgrosse]

Closed due to long inactivity. Feel free to reopen :)