Include xDAI / Polygon / L2 as bounty chains

### User Story

[comment]: # (As a , I want to so that .)
As a bounty creator, I want to payout bounty workers with tokens on xDAI and/or Polygon and/or L2 chains, so that small/medium size bounties can be created without suffering high gas price.

### Why Is this Needed

[comment]: # (Describe the problem and why this task is needed. Provide description of the current state, what you would like to happen, and what actually happen)
*Summary*: It's amazing that more and more chains are supported at Gitcoin. How are those chains selected?
Given many tokens have been bridged to xDAI / Polygon and a growing number to Arbitrum / Optimism, supporting those chains can encourage projects to fund small/medium size bounties and thus engage a broader community.