feat: Added functions to configure ca certificate to the global-agent

[vipendra-mehra]

Provided options to configure ca certificate to the global-agent

1. ca certificates can be set while initiating global-agent (using constructor)
2. ca certificate can be set/reset after initiation of global-agent (using methods)

Please review the code.

[vipendra-mehra]

@gajus These are final changes, please review them and let me know if your feedback

[vipendra-mehra]

@gajus did you get a chance to code review changes?