feat: migrate to the connect based grpc setup

[JamieSinn]

This PR

This is a pretty near 1:1 swap of the grpc handler to align to connect.

There are a few notable changes:

• the setting added in [FEATURE] Configurable gRPC keepalive enforcement policy on the flag-sync server #1998 are not required anymore because connect has no minimum.

This on the client side is a drop in replacement; no changes there needed.

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[erka]

great work! some small nits from me.

[toddbaert]

I would love if we could make /v1/flags and /flagd.sync.v1.FlagSyncService/FetchAllFlags the same endpoint literally but I don't think we can, due to the way the latter must be wrapped to match the proto payload (FetchAllFlagsResponse). This is the next best thing, I think (a human readable /v1/flags and a connect compatible /flagd.sync.v1.FlagSyncService/FetchAllFlags. It allows us to drop the cmux dep.

[toddbaert]

Follow-up, not for this PR: we could mark FetchAllFlags idempotency_level = NO_SIDE_EFFECTS so the Connect path is GET-able and CDN/proxy-cacheable. It's additive/non-breaking: server still serves POST and just also accepts GET; gRPC clients unaffected; Connect clients keep POSTing unless they opt into WithHTTPGet(). HTTP/1.1 clients can use both POST and GET.

Caching caveat: the selector must be in the URL (query/message) to be cache-safe, or the server sets Vary: Flagd-Selector on the response so caches key on that request header too... or it wouldn't be a cache factor.

I'll open an issue to track that I think.

[toddbaert]

Similar to @erka , only nits and notes. Great work.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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