client/http: expose approximate_kv_size in RegionInfo

[D3Hunter]

Enhancement Task

PD HTTP API region info includes approximate_kv_size, but client/http.RegionInfo currently drops this field during JSON decode.

This blocks downstream TiDB code paths that need to read region approximate kv size via the PD HTTP client.

Proposed change:

• Add ApproximateKvSize int64 json:"approximate_kv_size"`` to client/http.RegionInfo
• Add JSON decode tests for both present and missing field values (missing should fallback to zero value).