Fix TLS struct keys and adjust field tags to match field names

[zalegrala]

The previous update to enable TLS config settings leveraged existing structs which, when mapped from JS -> Golang, did not end up with a friendly user interface. Here we clean up the struct fields after reading the following docs.

https://grafana.com/docs/k6/latest/extensions/explanations/go-js-bridge/#about-the-go-to-js-bridge

Also update the docker examples to match the new struct keys.

[stoewer]

Nice PR. Tested it locally and works well.

There is also a configuration example in the README.md where we use the top level insecure: true. It should be changed to:

tls: {
   insecure: true,
}

[stoewer]

LGTM