net-mgmt/zabbix-proxy: add support for SSHKeyLocation option

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Is your feature request related to a problem? Please describe.
When using SSH monitoring commands in Zabbix, required SSH private/public keys must also be placed on the Zabbix proxy.

When SSHKeyLocation is not configured on the Zabbix proxy, the following error message is logged:

Authentication by public key failed. SSHKeyLocation option is not set

Describe the solution you'd like
Support for the SSHKeyLocation option should be added to the Zabbix proxy plugin. Furthermore, a possibility to add/manage/delete private and public SSH keys should also be added.

Describe alternatives you've considered
Not using SSH monitoring commands in Zabbix, but this would limit the usability of the Zabbix proxy plugin.

Additional context
I’m planning to work on it, if no one has any objections. 🙂