Add a `stdin` source to the viewer so it can stream data in from
standard input.

We make `stdin` an explicit input (`rerun -`) like in the good old days,
to avoid shady behavior in non-TTY and other esoteric environment (crazy
WSL setups, etc). Trust me, it gets quite insane...

Checks:
- [x] `rerun` works
- [x] `rerun -` blocks
- [x] `rerun - < data.rrd` works
- [x] `cat data.rrd | rerun -` works
- [x] `rerun < data.rrd` works but does nothing
- [x] `cat data.rrd | rerun` works but does nothing
- [x] no-TTY & other esoteric environments don't go completely off the
rails

---

Part of a small PR series to add stdio streaming support to our Viewer
and SDKs:
- #4511
- #4512 
- #4513
- #4514

…4512)

Allow the Rust SDK to stream RRD data to stdout.

Checks:
- [x] `just py-build && echo 'hello from stdin!' | cargo run -p stdio |
rerun -`

---

Part of a small PR series to add stdio streaming support to our Viewer
and SDKs:
- #4511
- #4512 
- #4513
- #4514

…4514)

Allow the C++ SDK to stream RRD data to stdout.

Checks:
- [x] `just py-build && just cpp-build-examples && echo 'hello from
stdin!' | ./build/debug/examples/cpp/stdio/example_stdio | rerun -`

---

Part of a small PR series to add stdio streaming support to our Viewer
and SDKs:
- #4511
- #4512 
- #4513
- #4514