Refactor init in Python SDK

[emilk]

We want to make the python API look more similar to the Rust and C++ APIs.

Exactly how needs a bit of design, but something along the lines of:

rec = rr.Recording("rerun_example_app", stream_to_disk="foo.rrd")
rec.set_time(…)
rec.log(…)

# Helper to have a standard way to access a recording:
rr.set_global_recording(rec)
if rec := rr.get_global_recording():
    rec.log(…)

No init. No other global state or per-thread state other than the set/get_global_recording helpers.

Questions: how do we handle multiprocessing parallelism?

Related:

• Defaulting the RecordingId to authkey() should be opt-in #3793
• Rethinking stateful time APIs to avoid surprising behaviors #3743
• PythonSession should always set recording-id when acquiring global session #1792
• python spawn=True says "trying to connect" on already open viewer but doesn't announce success #5175
• Support multiple sinks at once (tee logging) #1998
• Remove the "blueprint stream" API surface #5296
• Stateless time APIs #3841