Add support for OPA configs

[philipaconrad]

Description

As part of #57, we'd like to support parsing OPA config files.

Since YAML is not a supported encode/decode format out-of-box for Swift, we'll focus initially on correctly parsing JSON configs. We may later add support for using the YAML format for configs once the structural bits are plumbed in.

Tasks

The ⭐ symbol indicates this field is a priority to port over for most use cases.

• ⭐ Port over the services config section. (PR for the other 4x service credential types)
  • Port over services[_].oauth2 config section.
  • Port over services[_].s3_signing config section.
• ⭐ Port over the bundles config section.
• Port over the status config section.
• Port over the decision_logs config section.
• ⭐ Port over the discovery config section.
• ⭐ Port over the keys config section.
• Port over the caching config section.
• Port over the distributed_tracing config section.
• Port over the storage.disk config section.
• ⭐ Port over the server config section.
• Port over the miscellaneous config fields/sections.

[philipaconrad]

Okay, so after tangling with this for a few days, I've come to realize just how sprawling and massive OPA's configuration surface is. We also do a LOT of delegated/deferred parsing with json.RawMessage values, which has allowed us to scatter config logic to the four winds across the OPA codebase. Just gathering up and porting over all of the struct definitions and validation logic for the services section has been an adventure. 😅

As a result, I'm going to break the config porting effort up along config section lines, so that we can review and merge disjoint pieces. Hopefully that will keep the diffs smaller too. I'll add tasks to the top-level issue description to properly lay this out.

[philipaconrad]

I wound up porting over the overwhelming majority of these features in the Swift OPA SDK, as it was easier to prototype things over there, and we had no restrictions on adding libraries and external dependencies at all.

Now that we've upgraded to Swift 6.1, which introduces package traits, we may want to look into re-absorbing the config definitions from the Swift OPA SDK.