Refactor cli entry point and configuration

[Pokom]

One of the patterns that I think is making our startup process more difficult is how we specify the cloud provider and the specific metrics we want to scrape for the provider. Today we'd have something like:

./cloudcost-exporter -provider=${PROVIDER} [args]

The idea was to limit to a single cloud provider and then have arguments that were kind of specific to each one. However as the application has grown, I think this is fairly cumbersome both from the code perspective and using it. It means we can only target one CSP at any given time, and that's fine in production but can be cumbersome for local development. Ideally the application should run if it's able to.

Prometheus's instrumentating documentation provides a pretty good reference on what we should be aiming for:

When working with applications, you should aim for an exporter that requires no custom configuration by the user beyond telling it where the application is. You may also need to offer the ability to filter out certain metrics if they may be too granular and expensive on large setups, for example the HAProxy exporter allows filtering of per-server stats. Similarly, there may be expensive metrics that are disabled by default.

As such we should aim to simplify our configuration as much as possible.

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1. feat(cli): Create a generic Config struct #9
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[Pokom]

I'm going to split up this issue a bit since I think it was discussing two distinct problems:

1. CLI configuration and how it's handled in code
2. How to enable/disable collectors for each provider

I'll spin up a new issue to refactor how we setup collectors for each provider.