C++: Replace the many parameters of `rerun::spawn` with a `struct`

[kyle-figure]

Describe the annoyance
Overriding one default parameter in RecordingStream::spawn requires passing in all previous parameters:

// Spawn viewer with flush timeout of 10 seconds:
rec->spawn(9876, "75%", "rerun", std::nullopt, 10.0F).exit_on_failure();

Expected behavior
Be able to override a single default parameter without providing all previous default values.

Your goals
It would be nice to define the configs in a struct with defaults and pass it into the function that way. With this, we would be able to do:

// Spawn viewer with flush timeout of 10 seconds:
rec->spawn({.flush_timeout_sec=10.0}).exit_on_failure();

Desktop (please complete the following information):

• OS: Ubuntu 22.04 x86_64

[Wumpf]

ah good point, a parameter struct might be nicer here indeed! We don't get to use the fancy C++20 (or C99...) struct assignment in our examples for backwards compatibility, but it's still a win!

[Wumpf]

#4149 ironically doesn't move the flush_timeout_sec into the struct because rerun::spawn doesn't take it. But everything else and and I added a chrono overload. So you can do now:

using namespace std::chrono_literals;
// ...
rec.spawn({}, 4s);