feat(csharp): add opt-in SpanInputStream variants and OptimizedToken classes #1323

Job	Run time
cpp-lib-build (macos-15, clang, OFF)	3m 57s
cpp-lib-build (macos-15, gcc, ON)	50s
cpp-lib-build (macos-15, clang, ON)	51s
cpp-lib-build (ubuntu-22.04, clang, ON)	2m 52s
cpp-lib-build (ubuntu-22.04, clang, OFF)	4m 27s
cpp-lib-build (macos-15, gcc, OFF)	3m 15s
cpp-lib-build (ubuntu-22.04, gcc, ON)	1m 39s
cpp-lib-build (ubuntu-22.04, gcc, OFF)	6h 0m 15s
cpp-lib-build (windows-2022, cl)	3m 27s
cpp-lib-build (windows-2022, clang, OFF)	5m 26s
cpp-lib-build (windows-2022, clang, ON)	4m 27s
build (macos-15, cpp)	9m 7s
build (macos-15, dart)	5m 51s
build (macos-15, go)	2m 47s
build (macos-15, csharp)	4m 4s
build (macos-15, php)	1m 45s
build (macos-15, typescript)	8m 8s
build (macos-15, java)	1m 30s
build (ubuntu-22.04, go)	3m 45s
build (ubuntu-22.04, javascript)	2m 16s
build (macos-15, tool)	54s
build (windows-2022, cpp)	24m 22s
build (ubuntu-22.04, typescript)	9m 16s
build (macos-15, javascript)	1m 17s
build (ubuntu-22.04, python3)	1m 23s
build (windows-2022, csharp)	7m 52s
build (ubuntu-22.04, csharp)	6m 12s
build (ubuntu-22.04, java)	1m 20s
build (ubuntu-22.04, cpp)	19m 6s
build (windows-2022, tool)	2m 1s
build (macos-15, python3)	1m 14s
build (ubuntu-22.04, tool)	1m 11s
build (ubuntu-22.04, php)	1m 29s
build (windows-2022, go)	5m 29s
build (windows-2022, java)	1m 49s
build (windows-2022, dart)	4m 11s
build (windows-2022, typescript)	25m 29s
build (windows-2022, javascript)	2m 19s
build (ubuntu-22.04, dart)	3m 43s
build (windows-2022, php)	2m 3s
build (windows-2022, python3)	1m 55s
	9h 15m 14s

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