Disable verification of the proxied HTTPS server certificates

Is your feature request related to a problem? Please describe.
API ML validates SSL certificates of the southbound services. This feature is required for production deployment, but it's probably not necessary to be enabled by default.

Describe the solution you'd like
Disable verification of the proxied HTTPS server certificates by default and allow users to decide if they want to enable it.
This seems to be standard among gateways and proxies: https://developer.konghq.com/gateway/configuration/#proxy-server-ssl-verify or https://nginx.org/en/docs/http/ngx_http_proxy_module.html
Describe alternatives you've considered
Improve SSL troubleshooting and certificate management.

Willingness to help
If you are willing to help with feedback on the implementation or with the actual implementation,
please state your willingness to assist in the created issue. We will take it into account when figuring what to work on.

Additional context
Add any other context or screenshots about the feature request here.

The issue names no files, tests, or entry points. Start by locating API ML's southbound HTTPS certificate-validation configuration and reviewing how proxy SSL settings are documented; define the default and opt-in behavior, then verify it with the project's relevant SSL or proxy tests.