Add support for multiple ClickHouse configurations

[ZLBillShaw]

This PR adds the ability to support multiple ClickHouse configurations in the system. It introduces changes to config parsing and adjusts connection logic accordingly.

[CLAassistant]

All committers have signed the CLA.

[serprex]

This PR seems to contain changes outside the scope of its description

Multiconfig seems like a nonstandard convention which we would prefer not to go forward with. Could you describe your use case & the challenge you have adjusting environment variables based on your deployment environment?

[ZLBillShaw]

Thanks for the feedback!

Let me clarify the use case first:

In our deployment, we have multiple ClickHouse clusters across different environments and regions (e.g., per-tenant or per-datacenter deployments). We frequently need to query different clusters depending on the context, often in parallel or without restarting the collector.

The existing single-config model forces us to restart or redeploy with updated environment variables every time we want to switch ClickHouse targets, which is operationally inconvenient and not scalable for larger multi-tenant environments.

That's why we proposed a multi-config approach to support multiple configured ClickHouse targets that can be selected dynamically during runtime. This helps us run a single instance of the collector that can handle data routing to the appropriate ClickHouse without restarts or redeployments.