Allow use of firestore.transactional on instance methods

[andreas-thomik]

Is your feature request related to a problem? Please describe.
The current impementation of the @firestore.transactional decorator forces the transaction to be the first argument. This makes the wrapper unusable on methods which by design have self as their first argument.

Describe the solution you'd like
Ideally, the transaction object can be passed through keywords arguments. It should be possible to inspect the to_wrap function provided to check if a parameter named transaction exists. If not, fall back to the current implementation.

Describe alternatives you've considered
This can be worked around by using static methods, but makes for ugly code:

client = firestore.Client()

class MyClass:
    def __init__(self, a):
        self.a = a

    @staticmethod
    @firestore.transactional
    def _save_transactional(transaction, self):
        # Note: Not an actual "self" as normally understood in python code
        client.document("data/test").set({"a": self.a})
    
    def save(self):
        transaction = client.transaction()
        self._save_transactional(transaction, self)

Additional context
N/A