[feature] allow specification of the CA bundle via environment variable

[racinmat]

It would be useful to support global settings for the CA bundle in similar manner as e.g. python requests have it https://requests.readthedocs.io/en/latest/user/advanced/?highlight=REQUESTS_CA_BUNDLE#ssl-cert-verification using also the CURL_CA_BUNDLE env var would make it easier for other people who are dealing with CA bundles in other systems or languages.

I know now it can be done using

conf = SSLConfig()
MbedTLS.ca_chain!(conf, MbedTLS.crt_parse(cert_file))

or

HTTP.ConnectionPool.default_sslconfig = SSLConfig(...)

but it would be useful to have option which would not need code change.

[quinnj]

Sorry for the slowness; implemented in #933