Add support to specifying the configuration of the agent under test

[rapphil]

Right now the configuration in the compliance tests is hard coded in the test targets. Example: https://github.com/prometheus/compliance/blob/main/remote_write_sender/targets/otel.go#L16

It would be useful if it was possible to specify the configuration from a file using command line parameters.

If the configuration is not in a file, it should fall back to the to the hard coded version.

The reason why we need this is because depending on the version of the target under test, we need to use different configurations.