Asgiref tls extension proposal

[eirikhex]

Summary

Simplification of #1119 according to the discussion in django/asgiref#466
A proof of concept for a change in the asgi spec for the tls extension where
client_cert_name is removed because it's not the role of the ASGI web server to parse X.509 certificate. The content of the field is a part of the client_cert_chain, and should be extracted from there. (as shown in the test in this PR)
client_cert_error is removed as it was impossible to fill with any useful information.
tls_version and cipher_suite is filled with string returned by ssl.SSLSocket.version() and ssl.SSLSocket.cipher()

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.