Provide support for hot-swapping scraping configuration

[tomkerkhove]

Provide support for hot-swapping scraping configuration so that agent does not need to be restarted.

[brusMX]

This is a feature that our customers would like to have.

Because they need to be able to change the metrics on the fly without having to restart the service or re-deploy it.

They want it to be fully independent and decouple so it can live even if the service is down and continue to work even if the service is deployed after Promitor was deployed.

Also they want to make sure that Promitor will start fetching new metrics the moment metrics.config file is updated.

[tomkerkhove]

This will be done as part of #431 via #608 for runtime configuration but not for scraping configuration.

[tomkerkhove]

We should move the validation inside the deserialization somehow rather than based on the output - see @adamconnelly his suggestion

[github-actions]

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