QGrpcHttp2Channel: raise the default HTTP/2 receive window

A bare QHttp2Configuration advertises only a 64 KiB receive window for
both the stream and the connection. HTTP/2 flow control bounds the
unacknowledged data the peer may send, so download throughput is capped
at window/RTT: on a fast link the sender stalls waiting for
WINDOW_UPDATE frames and the pipe sits idle.

Default to a 4 MiB per-stream and 16 MiB per-connection receive window.
The connection window is kept the larger of the two so it never
bottlenecks multiplexed streams; both match the windows used by the C++
gRPC reference and Go's net/http2.

Measured with tests/manual/grpc/benchmarks (asyncbenchserver, 1000
calls, http loopback, arm64 macOS), stock vs. patched:

payload  RPC              before    after      delta
16 KiB   ServerStreaming  428 MB/s  1.10 GB/s  +157%
256 KiB  ServerStreaming  470 MB/s  2.68 GB/s  +471%
1 MiB    ServerStreaming  471 MB/s  2.98 GB/s  +533%  (2.23 s → 353 ms)
1 MiB    BidiStreaming    743 MB/s  2.31 GB/s  +211%  (2.82 s → 910 ms)
1 MiB    UnaryCall        747 MB/s  1.89 GB/s  +153%  (2.81 s → 1.11 s)

UnaryCall server→client response latency at 1 MiB drops from 2.19 ms to
454 µs (−79%). Small messages stay within run-to-run noise, and
upload-only ClientStreaming is unchanged because the client receive
window does not govern outbound data.

[ChangeLog][QtGrpc][QGrpcHttp2Channel] The default HTTP/2 receive window
was raised from 64 KiB to 4 MiB per stream and 16 MiB per connection.
The per-stream window can be tuned via the
QT_GRPC_HTTP2_STREAM_RECEIVE_WINDOW_SIZE environment variable and the
connection window via QT_GRPC_HTTP2_CONNECTION_RECEIVE_WINDOW_SIZE

Pick-to: 6.12 6.11 6.8
Fixes: QTBUG-136766
Change-Id: Ide6ae9dd941283519052632db7165583d1002a00
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>