refactor: cleanup export flags

[TheSpiritXIII]

This PR cleans up all export flags so that:

• The code is more readable as it's more broken up by function.
• We could move these options to YAML one day and update them at runtime instead of at startup.

Changes:

• Ensures all flag values have their own struct.
• Move flag validation and defaulting to their own methods.
• Ensure all flag structs have only serializable values (e.g. ExporterOpts.Lease was moved out).
• Use URLs with kingpin (flag library) to avoid needing to parse strings manually.

I also broke up the monstrosity that was the setup package exporter constructor which returned a function that would create an exporter. The reason this was done was because first we setup flags and then returned a method that would create the exporter once the flags were parsed. Now you can just call SetupFlags and then call exporter.New.

Tested a Prometheus version built with these changes with e2e tests, and they pass.

[TheSpiritXIII]

For ease of making this PR easier to review, I split up the rule-evaluator refactors to this PR: https://github.com/GoogleCloudPlatform/prometheus-engine/pull/1058