feat: Per Monitor DNS

[abrondijk]

Description

Add an option to specify what DNS resolver will be used for the monitor. Useful when you want to monitor internal instances, as well as external instances to make sure the instance is reachable to the outside world. Currently, I would need two instances of uptime-kuma to monitor both.

Fixes #1302

Example: I want to monitor the webUI of my NAS, which has a hostname that's only resolved through a local entry (resolves IP of local reverse proxy), but I also want to monitor some website that is resolvable both internally (resolves IP of local reverse proxy) and externally (resolves public IP).

Type of change

• User interface (UI)
• New feature (non-breaking change which adds functionality)

Checklist

• [ ] My code follows the style guidelines of this project
• [ ] I ran ESLint and other linters for modified files
• [ ] I have performed a self-review of my own code and tested it
• [ ] I have commented my code, particularly in hard-to-understand areas (including JSDoc for methods)
• [ ] My changes generate no new warnings
• [ ] My code needed automated testing. I have added them (this is optional task)

Screenshot