feat: parseConfiugration helper

[greghuels]

Summary

Introduces a new parseConfiguration() helper function that simplifies configuration setup for SDK users.

Example:

  auto result = eppoclient::parseConfiguration(flagConfigJson, banditModelsJson);
  if (!result.hasValue()) {
      std::cerr << "Configuration parsing failed:" << std::endl;
      for (const auto& error : result.errors) {
          std::cerr << "  - " << error << std::endl;
      }
      return 1;
  }
  configStore.setConfiguration(std::move(*result.value));

[dd-oleksii]

minor: we have a configuration wire format that combines both flags and bandits configuration in the same json. I wonder if we should reserve parseConfiguration for that.

(From public API perspective, I think we only want to expose this configuration wire parsing as this is our intended format for passing configuration to/from SDKs.)

[greghuels]

I think this is fine, since we're likely not going to need to pull in configuration wire functionality in this SDK

[dd-oleksii]

nit: we might provide another parseConfiguration override that only takes a single argument, so this "pass empty string" is not exposed to user

[greghuels]

I think that comment wasn't necessary to begin with, since it's an optional parameter. Though adding the override is probably a bit more clear, so I'll update.

[dd-oleksii]

nit: do we expect examples to run from ./examples directory? Not opposed to that but that seems inconsistent with tests, that run from repository root

[greghuels]

I think this makes more sense than updating the relative paths in the examples, yeah. These are meant to be self-contained examples that the user could copy if they wanted.