Expose API for TLS reqwest clients

This merge will add two new methods to the ClientBuilder struct each corresponding to one of two ways that the reqwest crate allows client Identities to be constructed. This is to support Gitlab instances that require HTTPS client authentication prior to and independent from Gitlab's token-based authentication methods.

These methods require additional features from reqwest, so are also gated and non-default.

The second commit on the branch makes these new feature gates visible in the generated documentation (especially docs.rs). It has been kept separate because it requires an unstable language feature (doc_cfg) and may need to be severed from the primary content of the MR.