feat(ldreason): add isOverride indicator to EvaluationReason

[kinyoklion]

Adds an optional boolean isOverride indicator to ldreason.EvaluationReason, identifying evaluations whose flag definition was supplied by an SDK override source rather than by LaunchDarkly. This is the go-sdk-common piece of the SDK flag-overrides feature (OVERRIDE spec).

• Unexported isOverride field with an IsOverride() accessor, following the inExperiment pattern.
• NewEvalReasonFromReasonWithIsOverride copy-style helper, mirroring NewEvalReasonFromReasonWithBigSegmentsStatus, since the indicator can apply to a reason of any kind.
• Serialized only when true, so the wire format is unchanged for all existing evaluations. One writer/reader change covers encoding/json, jsonstream, and easyjson (the easyjson methods delegate to the shared functions); an easyjson build-tagged round-trip test verifies that path.
• The serialization test table now also exercises every reason kind with the indicator set, and asserts that setting it to false serializes identically to today.

SDK-2651

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Note

Low Risk
Additive API and backward-compatible JSON; no evaluation or auth logic changed in this package.

Overview
Adds an optional isOverride flag on EvaluationReason so evaluation details can record when the evaluated flag came from an SDK override source instead of LaunchDarkly (OVERRIDE spec / SDK-2651).

IsOverride() and NewEvalReasonFromReasonWithIsOverride mirror inExperiment / NewEvalReasonFromReasonWithBigSegmentsStatus, so any reason kind can carry the indicator without changing its kind. JSON only emits isOverride when true, keeping the wire shape unchanged for existing payloads; shared jsonstream read/write covers encoding/json and easyjson.

Tests factor serialization cases into makeSerializationTestParams, cover override on every reason kind, and add an easyjson round-trip test behind launchdarkly_easyjson.

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