Allow using alternative TLS backends

[R1kaB3rN]

httpcore uses the ssl module provided by the standard library, which itself is dependent on OpenSSL. However, there are problems with OpenSSL, and it looks like it’ll be here to stay for the foreseeable future.

For some context, see https://discuss.python.org/t/our-future-with-openssl/21486/10

This might be out of scope for this project, but it would be nice if httpcore allowed users to optionally use a different backend (e.g., rustls). On that note, there are official Python bindings to rustls. See https://github.com/djc/pyrtls