Higher Trading Frequency in FinRL-meta

**Is your feature request related to a problem? Please describe.**
The majority of the functionalities in FinRL-meta are daily based, including technical `INDICATORS` and `StockTradingEnv`. If want to trade in higher frequency based on timestamp instead of days, where should be modified?

**Describe the solution you'd like**
Be able to trade based on timestamp instead of daily data

**Describe alternatives you've considered**
An ugly solution is to create fake _dates_ based on timestamp and convert back before plotting graphs.