refactor: use hooks to extend bulk_transaction (backport #43058)

[mergify]

Bulk Transaction is very useful feature, but now it is really difficult to add more methods.

When creating a new app, there is a chance that the the new kind of bulk transaction is required.

This PR move the mapper function inside a Doctype so it can be extened by super() in other app (I can't find a proper doctype other than Bulk Transaction Detail Log to place the get_mapper()).

I.e., I have a new app, and so I can do something like this with override_doctype_class.

from erpnext.bulk_transaction.doctype.bulk_transaction_log_detail.bulk_transaction_log_detail import BulkTransactionLogDetail


class BulkTransactionLogDetailThaiPayroll(BulkTransactionLogDetail):
    
    def get_mapper(self):
        from thai_payroll.custom import salary_slip
        mapper = super().get_mapper()
        mapper.update({
            "Salary Slip": {
		    	"Withholding Tax Cert Employee": salary_slip.make_withholding_tax_cert_employee,
		    },
        })
        return mapper

Note: IMHO, writing method within a class should be a preferred way to allow easier extension. Currently, in many case, I always need to do monkey patch and method overwrite (which is not good for upgrade).

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This is an automatic backport of pull request #43058 done by Mergify.