ccl/sqlproxyccl: add benchmarks for connection copying logic

Informs cockroachdb#76000. Follow up to cockroachdb#76006.

This commit adds a benchmark test for the connection copying logic.

```
    go version go1.17.3 linux/amd64
    ---
    goos: linux
    goarch: amd64
    pkg: github.com/cockroachdb/cockroach/pkg/ccl/sqlproxyccl/interceptor
    cpu: AMD Ryzen 9 5950X 16-Core Processor
    BenchmarkConnectionCopy/msgCount=10000/msgLen=170/io.Copy-32                   385      3538410 ns/op
    BenchmarkConnectionCopy/msgCount=10000/msgLen=170/io.CopyN-32                  15      75036404 ns/op
    BenchmarkConnectionCopy/msgCount=10000/msgLen=170/pgproto3-32                  26      42251150 ns/op
    BenchmarkConnectionCopy/msgCount=10000/msgLen=170/chunkreader-32               18      65000047 ns/op
    BenchmarkConnectionCopy/msgCount=10000/msgLen=170/interceptor-32               33      39125341 ns/op

    BenchmarkConnectionCopy/msgCount=500/msgLen=320/io.Copy-32                   3204        406888 ns/op
    BenchmarkConnectionCopy/msgCount=500/msgLen=320/io.CopyN-32                   410       3123298 ns/op
    BenchmarkConnectionCopy/msgCount=500/msgLen=320/pgproto3-32                   577       2387535 ns/op
    BenchmarkConnectionCopy/msgCount=500/msgLen=320/chunkreader-32                469       3565173 ns/op
    BenchmarkConnectionCopy/msgCount=500/msgLen=320/interceptor-32                652       2015079 ns/op

    BenchmarkConnectionCopy/msgCount=9000/msgLen=2900/io.Copy-32                   49      23330567 ns/op
    BenchmarkConnectionCopy/msgCount=9000/msgLen=2900/io.CopyN-32                  18      72003323 ns/op
    BenchmarkConnectionCopy/msgCount=9000/msgLen=2900/pgproto3-32                  15      82500818 ns/op
    BenchmarkConnectionCopy/msgCount=9000/msgLen=2900/chunkreader-32               18      79832494 ns/op
    BenchmarkConnectionCopy/msgCount=9000/msgLen=2900/interceptor-32               20      54727023 ns/op

    BenchmarkConnectionCopy/msgCount=5000/msgLen=30000/io.Copy-32                  12      98640876 ns/op
    BenchmarkConnectionCopy/msgCount=5000/msgLen=30000/io.CopyN-32                 10     110690053 ns/op
    BenchmarkConnectionCopy/msgCount=5000/msgLen=30000/pgproto3-32                  6     177894915 ns/op
    BenchmarkConnectionCopy/msgCount=5000/msgLen=30000/chunkreader-32               9     129588686 ns/op
    BenchmarkConnectionCopy/msgCount=5000/msgLen=30000/interceptor-32               9     112610362 ns/op

    BenchmarkConnectionCopy/msgCount=10000/msgLen=10/io.Copy-32                   591       2274817 ns/op
    BenchmarkConnectionCopy/msgCount=10000/msgLen=10/io.CopyN-32                   25      47465099 ns/op
    BenchmarkConnectionCopy/msgCount=10000/msgLen=10/pgproto3-32                   58      23077900 ns/op
    BenchmarkConnectionCopy/msgCount=10000/msgLen=10/chunkreader-32                38      31459201 ns/op
    BenchmarkConnectionCopy/msgCount=10000/msgLen=10/interceptor-32                64      17616468 ns/op

    BenchmarkConnectionCopy/msgCount=10000/msgLen=15/io.Copy-32                   531       2336896 ns/op
    BenchmarkConnectionCopy/msgCount=10000/msgLen=15/io.CopyN-32                   30      45135200 ns/op
    BenchmarkConnectionCopy/msgCount=10000/msgLen=15/pgproto3-32                   51      22100293 ns/op
    BenchmarkConnectionCopy/msgCount=10000/msgLen=15/chunkreader-32                49      28931167 ns/op
    BenchmarkConnectionCopy/msgCount=10000/msgLen=15/interceptor-32                66      15189020 ns/op
```

Release note: None

pkg/ccl/sqlproxyccl/interceptor/BUILD.bazel

@@ -14,12 +14,20 @@ go_library(
 
 go_test(
     name = "interceptor_test",
-    srcs = ["interceptor_test.go"],
+    srcs = [
+        "bench_test.go",
+        "interceptor_test.go",
+    ],
     embed = [":interceptor"],
     deps = [
         "//pkg/sql/pgwire/pgwirebase",
+        "//pkg/util/grpcutil",
         "//pkg/util/leaktest",
+        "//pkg/util/log",
+        "//pkg/util/stop",
         "@com_github_cockroachdb_errors//:errors",
+        "@com_github_cockroachdb_logtags//:logtags",
+        "@com_github_jackc_chunkreader_v2//:chunkreader",
         "@com_github_jackc_pgproto3_v2//:pgproto3",
         "@com_github_stretchr_testify//require",
     ],