Add `is_required` / `is_repeated` properties to `FieldDescriptor` (#1… #4177

Job	Run time
Check scripts and tests with mypy (linux)	18s
Check scripts and tests with pyright (Linux)	20s
Check scripts and tests with pyright (Windows)	18s
Stubsabot dry run	27s
Check scripts and tests with mypy (win32)	20s
	1m 43s

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