What is the maximum taxi speed and who defines it? Which is the maximum speed that an airliner is limited to while taxiing?
I know that during 90 degree turns it is 10 knots, but what about long, straight taxiways? 20 knots? 25? 30? 

More broadly, where I can find this information?
Would it be in company SOPs? Manufacturer's FCOM?
And here, a Reference from Boeing's 737 FCTM (pdf). Ch2 for ground operations.