Implement fetching account statements (HKKAU, HKEKA)

[johan12345]

This adds support for listing and downloading account statements (Kontoauszüge, e.g., as PDF) using HKKAU and HKEKA segments.

Example:

from fints.client import FinTS3PinTanClient
from fints.formals import StatementFormat
from pathlib import Path

f = FinTS3PinTanClient(
    # ...
)
account = f.get_sepa_accounts()[0]
statements = f.get_statements(account)

for statement in statements:
    output_pdf = Path(f'statement_{statement.year}-{statement.statement_number:02d}.pdf')
    if not output_pdf.exists():
        resp = f.get_statement(account, statement.statement_number, statement.year, StatementFormat.PDF)
        with open(output_pdf, 'wb') as file:
            file.write(resp.data)