introduce HTTPX_CA_BUNDLE, fix docs

[Yanpas]

Summary

HTTPX_CA_BUNDLE env-variable was introduced (similar to REQUESTS_CA_BUNDLE). The motivation behind it: SSL_CERT_FILE affects other libraries/programs.

Alternatively I can introduce HTTPX_SYSTEM_CA.

Related to #302

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.

[Yanpas]

Also current documentation is misleading

[salman1993]

httpx docs suggest that they do not automatically pull in the environment variables SSL_CERT_FILE -
https://www.python-httpx.org/advanced/ssl/#working-with-ssl_cert_file-and-ssl_cert_dir

the code suggests that those env vars do get picked up:

httpx/httpx/_config.py

Lines 33 to 37 in e70d0b0

	if verify is True:
	if trust_env and os.environ.get("SSL_CERT_FILE"): # pragma: nocover
	ctx = ssl.create_default_context(cafile=os.environ["SSL_CERT_FILE"])
	elif trust_env and os.environ.get("SSL_CERT_DIR"): # pragma: nocover
	ctx = ssl.create_default_context(capath=os.environ["SSL_CERT_DIR"])