The HTTP/2 framer in gRPC uses a `bufio.Reader` with a 32KB buffer by
default. When there are a large number of transports, these buffers
consume significant memory, even when the transport is idle.

reads. This PR replaces the standard `bufio.Reader` with a custom
`io.Reader` implementation that uses pooled buffers and releases the
buffer once all data is consumed.

To defer the re-allocation of the read buffer, the reader calls
`ReadOnReady` on the underlying `io.Reader`. For this to work, the
underlying `io.Reader` must implement either the `ReadyReader` interface
or `syscall.RawConn`. If neither condition is met, the framer gracefully
falls back to using the regular `bufio.Reader`.

Additional Changes:
* The ALTS connection has been refactored to implement the `ReadyReader`
interface.
* The write buffer pools used by the framer are updated to use the
`mem.BufferPool` interface, allowing the pools to be shared across both
read and write operations.
* Use `syscall.Read` instead of `unix.Read` to avoid triggering the race
detector, see comment for details.
* Add environment variable protection for the changes to allow fast
rollback.

In a [real-world
benchmark](https://github.com/arjan-bal/custom-go-client-benchmark/tree/retry-dp),
where a GCS directpath client downloads a file in a loop, the average
"in use" memory falls from 28.3MB to 21.3MB (-24%).

Local Benchmarks show no significant difference
```
❯ go run benchmark/benchresult/main.go streaming-before streaming-after
streaming-networkMode_Local-bufConn_false-keepalive_false-benchTime_2m0s-trace_false-latency_0s-kbps_0-MTU_0-maxConcurrentCa
lls_120-reqSize_1024B-respSize_1024B-compressor_off-channelz_false-preloader_false-clientReadBufferSize_-1-clientWriteBuffer
Size_-1-serverReadBufferSize_-1-serverWriteBufferSize_-1-sleepBetweenRPCs_0s-connections_1-recvBufferPool_simple-sharedWrite
Buffer_true
               Title       Before        After Percentage
            TotalOps     29981273     29966908    -0.05%
             SendOps            0            0      NaN%
             RecvOps            0            0      NaN%
            Bytes/op      4971.06      4971.41     0.00%
           Allocs/op        19.79        19.79     0.00%
             ReqT/op 2046721570.13 2045740919.47    -0.05%
            RespT/op 2046721570.13 2045740919.47    -0.05%
            50th-Lat    461.523µs    460.906µs    -0.13%
            90th-Lat    654.435µs    655.327µs     0.14%
            99th-Lat   1.225856ms   1.240984ms     1.23%
             Avg-Lat    478.845µs    479.553µs     0.15%
           GoVersion     go1.25.0     go1.25.0
         GrpcVersion   1.81.0-dev   1.81.0-dev
```

RELEASE NOTES:
* transport: Pool HTTP/2 framer read buffers to reduce idle memory
consumption. Currently limited to Linux for ALTS and non-encrypted
transports (TCP, Unix). To disable, set
`GRPC_GO_EXPERIMENTAL_HTTP_FRAMER_READ_BUFFER_POOLING=false` and report
any issues.