Feature request: login with context

[willoma]

A zabbix.NewWithContext(ctx context.Context) could allow providing a context, that would be stored in the zabbix.Context object and used in httpPost, by calling http.NewRequestWithContext instead of http.NewRequest. Or maybe adding a WithContext method on the zabbix.Context strect.

I especially think of a context.WithTimeout that would provide the ability to fail if Zabbix is too slow to respond or when it is unreachable in certain circumstances.