Feature: Add getters in exchange

[jinyang1994]

Background

Sometimes you need to use FirstTradedTimestamp, LastSettledTimestamp, and LastFundingGrowthGlobal for statistics. Although I can find them through contract events, I think it would be very convenient if you can use some getters, so I added the following functions:

• Exchange.getFirstTradedTimestamp
• Exchange.getLastSettledTimestamp
• Exchange.getLastFundingGrowthGlobal

PR Reminders

Be ware of the followings

• implement deployment script for your changes in deployment repo
• add verification in hardhat simulation(000-prepare-simulation-check, 902-newMarket-check or 903-simulation-check) and system test(901-system-test) in deployment repo
• update change log

---

1. Contract: make sure the code follows our convention, ex: naming, explicit returns, etc.; if uncertain, discuss with others

2. Test: make sure tests can cover most normal and edge cases; if not, open follow-up tickets. Also, look out for failed tests and fix them!

3. CHANGELOG.md: update when external interfaces are changed

4. Workflow:

   • Github: assign the pr to yourself; if pairing, can merge directly; else, assign someone to help review
   • Asana: assign the corresponding ticket to yourself and leave the pr link on it for easier follow-ups
   • Discord: if someone is mentioned in this pr, tag on Discord

[jinyang1994]

@42bchen @tailingchen Can you review it? Thx