Add support for X509_PROVIDED

[f-blass]

The buildpack downloads the Authorization Bundle and authenticates with the Identity certificate.
In case of the credential type X509_GENERATED the Identity Broker generates a certificate and key and puts it into the binding.

In case of X509_PROVIDED this is not the case and the application needs a way to provide the private key (and cert) to the application.
This could be done via env variable, user-provided service, something else…?

This is quite easy to implement. The cert is still contained in the binding, only the key should be provided in another way. The challenge is to align with other components in providing this key. If anyone has an idea, please let us know

[nenaraab]

I have no answer on how applications normally shall provide the key to the environment, but we should also support external keys as part of our go client lib.