refact: use protoreflect to convert check request v3

[regeda]

The plugin transforms a CheckRequest into OPA's input in two steps:

1. protojson.Marshal
2. json.Unmarshal

Since CheckRequest is a protobuf type, OPA's input can also be generated using protobuf reflection.

Benchmark results.

goos: darwin
goarch: arm64
pkg: github.com/open-policy-agent/opa-envoy-plugin/internal
cpu: Apple M2 Pro

Using JSON as the intermediate format:

BenchmarkCheck-12          10000            116460 ns/op           87881 B/op       1734 allocs/op

Using protobuf reflection:

BenchmarkCheck-12          10000            103498 ns/op           80146 B/op       1592 allocs/op

[srenatus]

Thanks! LGTM, but let's make sure the changelog is clear about this improvement only applying to the v3 version of the API. (Which I assume, by now, most people should be using...?)

[regeda]

Thanks! LGTM, but let's make sure the changelog is clear about this improvement only applying to the v3 version of the API. (Which I assume, by now, most people should be using...?)

@srenatus I've updated the PR's title and the commit message. Should I update anything else?