Allow per-instance filesystem protocols

[be-student]

Closes #1800

AbstractFileSystem.protocol is currently declared as a ClassVar, so type checkers reject filesystem adapters that choose their protocol from constructor state. This removes the ClassVar restriction while preserving the existing string-or-tuple type and class default.

Concrete implementations with a fixed protocol can continue declaring their own ClassVar. The changelog now documents support for per-instance protocol selection.

Validation:

• The issue-shaped Pyright fixture fails before the change with Attribute "protocol" cannot be assigned through a class instance because it is a ClassVar and passes afterward with 0 errors.
• fsspec/tests/test_spec.py portable tests: 186 passed, 10 skipped, 1 xfailed. The 52 excluded Bash-reference cases require GNU stat -c and fail unchanged on macOS.
• Ruff check, Ruff format check, and git diff --check pass.

[martindurant]

OK, I suppose so; this is typing getting things wrong. It it a class variable usually....