Federal regulators are balking at a Massachusetts proposal to increase Medicaid spending by up to $1 billion a year over the next three years, prompting the state to request another two-week extension of its healthcare funding package. The federal payments, which are crucial for keeping the state's landmark health insurance law afloat, were set to expire June 30. But the state has received three extensions, and a fourth would postpone the deadline for reaching an agreement until Aug. 25. At stake in the negotiations between state and federal officials is more than $11 billion in federal funds over three years, money earmarked for dozens of healthcare programs in Massachusetts.