perf: Delay SignalRelay connection to when a callback is connected

[tlambert03]

closes #276

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e9ee496) 100.00% compared to head (4fdec36) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #277   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           23        23           
  Lines         1906      1926   +20     
=========================================
+ Hits          1906      1926   +20     

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[codspeed-hq]

CodSpeed Performance Report

Merging #277 will degrade performances by 11.53%

_{Comparing tlambert03:delayed-relay (4fdec36) with main (e9ee496)}

Summary

⚡ 4 improvements
❌ 1 (👁 1) regressions
✅ 61 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	tlambert03:delayed-relay	Change
⚡	test_dataclass_group_create[attrs]	2.8 ms	1.7 ms	+61.73%
⚡	test_dataclass_group_create[dataclass]	2.4 ms	1.3 ms	+79.77%
⚡	test_dataclass_group_create[msgspec]	2.8 ms	1.8 ms	+54.01%
⚡	test_dataclass_group_create[pydantic]	3.1 ms	1.7 ms	+81.13%
👁	test_emit_time[lambda-2]	109.4 µs	123.6 µs	-11.53%

[Czaki]

Generally looks ok. One suggestion and one question about the possibility of small improvement.