Add support for reading custom cert paths

[Owen-CH-Leung]

Fixes #103

As per discussed in #103 , this PR adds support to use custom ca cert via setting the env variable REQUESTS_CA_BUNDLE or CURL_CA_BUNDLE that points to the custom cert file

[Jake-Shadle]

I assume these variables are commonly set by those who use custom certs? If it is I guess this is fine, but personally I find them quite confusing since lib/curl nor requests (I guess python?) is used in any way by this library.

[Owen-CH-Leung]

I assume these variables are commonly set by those who use custom certs? If it is I guess this is fine, but personally I find them quite confusing since lib/curl nor requests (I guess python?) is used in any way by this library.

Yes. To my knowledge, REQUESTS_CA_BUNDLE is commonly used in python while CURL_CA_BUNDLE is commonly used when using the curl command line tool.

https://stackoverflow.com/questions/42982143/python-requests-how-to-use-system-ca-certificates-debian-ubuntu
https://curl.se/docs/sslcerts.html

I guess we can also use SSL_CERT_FILE which is commonly used for openssl. What do you think ?

https://www.openssl.org/docs/man1.1.1/man3/SSL_CTX_set_default_verify_paths.html

[Jake-Shadle]

I guess as long as they are documented it doesn't really matter

[messense]

I believe rustls isn't required here, only rustls-pemfile is needed.