Add HTTP3 support

[karpetrosyan]

This pull request tries to add HTTP/3 support.

As we know, the HTTP/2 and HTTP/3 protocols are very similar, except for the protocol they use. This PR simply follows the steps described below.

• Add the connect_udp method to "httpcore._backends.base.NetworkBackend".
• Implement connect_udp only for the synchronous backend (only for now).
• Add http3 extra into pyproject.toml
• Create httpcore/_http3.py file
• Implement HTTP/3 in that file, keeping the logic and flow maximum similar to the logic that we are using in _http2.py.

To support the HTTP/3 protocol, we need the aioquic package, which is a well-tested and well-designed implementation for the HTTP/3 and QUIC protocols.

For more details, see the issue in HTTPX, where the author of aioquic provides basic HTTP/3 integration for httpx.

There is a very basic example of how you can use HTTP/3 with the httpcore.

from httpcore import Origin
from httpcore import Request
from httpcore import HTTP3Connection
from httpcore import SyncBackend

host = "www.youtube.com"
port = 443

stream = SyncBackend().connect_udp(host=host, port=port)
conn = HTTP3Connection(
    origin=Origin(b"https", host.encode(), port), stream=stream
)

request = Request(
    method=b"GET",
    url=f"https://{host}",
    headers=[("host", host)],
    extensions={"timeout": {"read": 5, "write": 5}},
)


response = conn.handle_request(request=request)
print(response)   # <Response [200]>
print(response.extensions["http_version"])  # b'HTTP/3'
print(response.read())  # ...

Or with the high-level API:

import httpcore

pool = httpcore.ConnectionPool(http1=False, http3=True)

response = pool.request(
    "GET", "https://www.youtube.com", extensions={"timeout": {"read": 5, "write": 5}}
)

print(response)  # <Response [200]>
print(response.extensions["http_version"])  # b'HTTP/3'
print(response.read())  # ...