fix: prevent failures when unable to connect to *.nagios.com

[cxdy]

Fixes #50

• Creates an HTTP client with a specified timeout to catch 'connection timeout' errors instead of only waiting for an HTTP status code
• Skip updating the metric if a version isn't returned from GetLatestNagiosXIVersion

Tested on our Nagios development instance:

root@nagiosxi-1-dev:~/cody/nagios_exporter# ./nagios_exporter --nagios.check-updates  --nagios.scrape-uri http://localhost --nagios.timeout 60
INFO[0000] Using connection endpoint: http://localhost
WARN[0031] Nagios version wasn't found, not updating `nagios_update_available_info` metric
INFO[0034] Endpoint scraped and metrics updated
WARN[0071] Get "https://assets.nagios.com/downloads/nagiosxi/versions.php": dial tcp [2600:3c00::f03c:92ff:fef7:45ce]:443: connect: connection refused
WARN[0071] Nagios version wasn't found, not updating `nagios_update_available_info` metric
INFO[0074] Endpoint scraped and metrics updated
WARN[0121] Get "https://assets.nagios.com/downloads/nagiosxi/versions.php": dial tcp [2600:3c00::f03c:92ff:fef7:45ce]:443: connect: connection refused
WARN[0121] Nagios version wasn't found, not updating `nagios_update_available_info` metric
INFO[0124] Endpoint scraped and metrics updated
WARN[0161] Nagios version wasn't found, not updating `nagios_update_available_info` metric
INFO[0164] Endpoint scraped and metrics updated
WARN[0211] Nagios version wasn't found, not updating `nagios_update_available_info` metric
INFO[0214] Endpoint scraped and metrics updated
WARN[0251] Nagios version wasn't found, not updating `nagios_update_available_info` metric
INFO[0254] Endpoint scraped and metrics updated
WARN[0301] Nagios version wasn't found, not updating `nagios_update_available_info` metric
INFO[0304] Endpoint scraped and metrics updated
WARN[0341] Nagios version wasn't found, not updating `nagios_update_available_info` metric
INFO[0344] Endpoint scraped and metrics updated

[cxdy]

Confirmed this continues working properly after Nagios fixes their web server 😆

ime="2024-10-19T12:26:22-04:00" level=warning msg="Nagios version wasn't found, not updating `nagios_update_available_info` metric"
ime="2024-10-19T12:26:25-04:00" level=info msg="Endpoint scraped and metrics updated"
ime="2024-10-19T12:27:02-04:00" level=warning msg="Nagios version wasn't found, not updating `nagios_update_available_info` metric"
ime="2024-10-19T12:27:05-04:00" level=info msg="Endpoint scraped and metrics updated"
ime="2024-10-19T12:27:55-04:00" level=info msg="Endpoint scraped and metrics updated"
ime="2024-10-19T12:28:35-04:00" level=info msg="Endpoint scraped and metrics updated"
ime="2024-10-19T12:29:25-04:00" level=info msg="Endpoint scraped and metrics updated"
ime="2024-10-19T12:30:05-04:00" level=info msg="Endpoint scraped and metrics updated"