Crystal's datasheet give load capacitance is 20pF, do I need external load capacitors?
All your questions are answered in AN2867 - Oscillator design guide for STM8AF/AL/S and STM32 microcontrollers. The link is terribly long, so if it didn't work correctly then go to www.st.com and search for "AN2867".

To make the read a bit easier, here are the quick answers to your questions:


  1/ Do I need to add 2 external load capacitors? If do not, where is crystal's ground pin like STM32F0's datasheet crystal section below


Yes you will need to add those two capacitors externally.


  2/ Can I know crystal's ESR to calculate Rext? Datasheet give maximum ESR value, so how I determine ESR value?


This app note tells you how to calculate Rext. And thankfully they use 8MHz as their example. From page 16 of the app note:


  An initial estimation of RExt is obtained by considering the voltage divider formed by RExt and CL2. Thus, the value of RExt is equal to the reactance of CL2. Therefore RExt = 1 / (2 π F CL2), and so, with an oscillation frequency of 8 MHz and
  CL2 = 15 pF, we have RExt = 1326 Ω.


And I would strongly recommend you read sections 7.1 and 7.2 (layout recommendations and examples) and follow them very closely. It can save you a lot of trouble.