[Security] Insecure TLS Versions must be explicitly whitelisted

[abhinavsingh]

Is your feature request related to a problem? Please describe.
Currently proxy.py allows insecure TLS protocols by default.
See https://github.com/abhinavsingh/proxy.py/security/code-scanning/6?query=ref%3Arefs%2Fheads%2Fdevelop for some details.

Describe the solution you'd like
Insecure TLS protocols (when required) must be explicitly whitelisted via flags or at runtime.