Add support for nested transactions

fsspec.transaction.Transaction() context manager should support nested transactions.
example usage:

def func1():
    fs = fsspec.filesystem('file')
    with fs.transaction:
        #write some files
        ...
        func2(fs)

def func2(fs):
    #this function does not know if the caller function already started a transaction on fs or not
    with fs.transaction:
        #write some other files
        ...

in the current implementation the transaction will complete (commit) once the with block in func2() completes

Start with fsspec.transaction.Transaction and trace how entering and exiting the context manager currently commits. Define the behavior for an inner transaction and an outer transaction, then verify that the example keeps the transaction open until the outer with block exits.