Add ContinuousDiD estimator (Callaway, Goodman-Bacon & Sant'Anna 2024) #301

Job	Run time
Rust Unit Tests (windows-latest)	2m 15s
Rust Unit Tests (macos-latest)	1m 18s
Rust Unit Tests (ubuntu-latest)	1m 20s
Pure Python Fallback	51m 27s
Python Tests (windows-latest, py3.11)	10m 32s
Python Tests (macos-latest, py3.13)	5m 47s
Python Tests (ubuntu-latest, py3.11)	9m 20s
Python Tests (ubuntu-latest, py3.13)	9m 59s
Python Tests (windows-latest, py3.13)	12m 52s
Python Tests (macos-latest, py3.11)	5m 9s
	1h 49m 59s

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