Make crypto provider configurable, rename feature and make hyper-tls feature visible

[threema-donat]

@dermesser tested with the four feature combinations

• "hyper-rustls", "service-account", "ring"
• "hyper-rustls", "service-account", "aws-lc-rs"
• "hyper-tls", "service-account", "ring"
• "hyper-tls", "service-account", "aws-lc-rs"

The test with the file seems to be flaky, could you please rerun this test?

It would be awesome to have a release with this change, so that we could start using the version with hyper v1.

[dermesser]

Thank you for taking the time to clean this up, it's much needed! I will merge and release soon, I just left a minor comment on the tests.