[WIP] Allow to select client CA certificate

[Zogoo]

TODO

1. Add test
2. Make it able to set SSL client verify mode.

[Zogoo]

@matsumotory I have created my idea level of pull request, I need your suggestion to how make this pull request as complete. I'm not sure, does it even work like this? Because, I'm newbie to this NGINX plugin.

Could you please take a look at it and tell what I need to do beside of the test.

Thanks.

Update: I could confirm that I can switch client CA certificate.