The Pants native client which was introduced in pantsbuild#11922 has so far only
been usable in the Pants repo when the `USE_NATIVE_PANTS` environment
variable was set.

To make the native client available to end users, this change begins
distributing the native-client binary in Pants wheels. A corresponding
change in the `scie-pants` repo
(pantsbuild/scie-pants#172) uses the native
client to run `pants`.

To reduce the surface area of `scie-pants` (rather than having it be
responsible for handling the special `75` exit code similar to the
`pants` script integration), this PR also moves to having the
native-client execute its own fallback (via `execv`) to the Pants
entrypoint. In future, the `pantsbuild/pants` `pants` script could also
use that facility (e.g. by specifying a separate `pants_server` bash
script as the entrypoint to use as the `_PANTS_SERVER_EXE`).

----

As originally demonstrated on pantsbuild#11831, the native client is still much
faster than the legacy client. Using
pantsbuild/scie-pants#172, the timings look
like:
```
Benchmark #1: PANTS_NO_NATIVE_CLIENT=true PANTS_SHA=836cceb74e6af042e7c82677f3ceb4927efce20e scie-pants-macos-x86_64 help
  Time (mean ± σ):      1.161 s ±  0.067 s    [User: 830.6 ms, System: 79.2 ms]
  Range (min … max):    1.054 s …  1.309 s    10 runs

Benchmark #2: PANTS_SHA=836cceb74e6af042e7c82677f3ceb4927efce20e scie-pants-macos-x86_64 help
  Time (mean ± σ):     271.0 ms ±  30.6 ms    [User: 8.9 ms, System: 6.9 ms]
  Range (min … max):   241.5 ms … 360.6 ms    12 runs

Summary
  'PANTS_SHA=836cceb74e6af042e7c82677f3ceb4927efce20e scie-pants-macos-x86_64 help' ran
    4.29 ± 0.54 times faster than 'PANTS_NO_NATIVE_CLIENT=true PANTS_SHA=836cceb74e6af042e7c82677f3ceb4927efce20e scie-pants-macos-x86_64 help'
```

Fixes pantsbuild#11831.