Ported from https://github.com/securedimensions/QGIS-OAuth2-Plugin
The Testbed 13 version provides an additional configuration tab "software statement" which allows a user to automatically register the plugin with a required configuration with the Authorization Server. Of course this can only be leveraged, if the Authorization Server involved supports the registration via digitally signed software statements (JWTs) as described in this ER.
See description of QgsAuthCertUtils::pkcs8PrivateKey.
This fix may be needed on other platforms (untested at this point),
because Qt5 QSslkey class *still* does not directly support creation
using non-PKCS1 PEM- or DER-encoded data, though QCA, whose qca-ossl
plugin is linked to OpenSSL, does support PKCS1 and PKCS8.
