Handle starting balances for bank sync

[bvanelli]

Following #25 , if syncing with a bank from scratch, the final amount might not match the balances, so updating the starting balances might be required.

[sondregronas]

Adding this here since it's such a minor thing:

I noticed that the Optional reference_date is actually required:

actualpy/actual/api/bank_sync.py

Line 63 in 65d9ea1

reference_date: Optional[str] = Field(..., alias="referenceDate", description="The date of the balance")

Setting default value to None works (GoCardless bank syncs was complaining about this missing)

reference_date: Optional[str] = Field(None, alias="referenceDate", description="The date of the balance")