[FEATURE] Respect SSL certificate environment variables

[aiudirog]

Is your feature request related to a problem?
I can't configure SSL cert files with the urllib3 backend through environment variables because certifi is chosen by default here:

opensearch-py/opensearchpy/connection/http_urllib3.py

Line 56 in cfe1190

CA_CERTS = certifi.where()

This is preventing me from configuring a company SSL certificate on the fly when deploying to Lambda which isn't required in other deployments.

What solution would you like?
Check SSL_CERT_FILE/SSL_CERT_DIR environment variables before defaulting to certifi's CA bundle.

What alternatives have you considered?
You can configure it manually in code, but this makes portable deployments more challenging.

Do you have any additional context?
N/A

[wbeckler]

@aiudirog Feel free to create a PR for this.