refactor: define new Synchronizer and Initializer interfaces for FDv2

[beekld]

This PR introduces IFDv2Initializer and IFDv2Synchronizer interfaces.

I based this on the Java design, but with some significant changes:

FDv2SourceResult Next(std::chrono::milliseconds timeout,
                      data_model::Selector selector) 

1. I made this a blocking call with a timeout, because C++ doesn't have any equivalent to "anyOf" for waiting on multiple racing promises, and this was the cleanest alternative I could come up with.
2. It doesn't have the same "conditions" structure as Java, but it seems like that's only used to implement timeouts, so I think we don't need it, unless I missed something.
3. Rather than introduce a SelectorProvider interface for the initializers/synchronizers to use, I am passing the selector directly into Next. I think this is acceptable, as Next only return a single result, which I think will only use one selector. This avoids having to deal with the ownership complications a shared provider would entail.

---

Note

Low Risk
Low risk because it only adds new FDv2 interface headers and a result type without changing existing runtime behavior; main risk is future integration/implementation mismatches around the blocking Next() timeout/selector contract.

Overview
Introduces new FDv2 source abstractions for the server SDK: a one-shot IFDv2Initializer (blocking Run() + Close()) and a continuous IFDv2Synchronizer with a blocking Next(timeout, selector) API.

Adds FDv2SourceResult as a unified variant return type covering successful changesets, transient/terminal errors, shutdown, goodbye/rotation signals, and timeout-based wakeups (including an fdv1_fallback flag where applicable). Also updates .gitignore to ignore .cache.

^{Written by Cursor Bugbot for commit ed22857. This will update automatically on new commits. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, have a team admin enable autofix in the Cursor dashboard.}

[kinyoklion]

This is nicer than the Java shenanigans.