integrate layer 2 ETH solutions for bounty payments

**User Story:**
As a project initiator and bounty creator,
I want to use layer 2 solutions for paying bounties,
So that I can avoid spending the little money I have on ETH gas fees.

**Tech Details:**
implement payment of contributors via:
* polygon
* gnosis chain

**Open Questions:**

**Notes/Assumptions:**
created from ticket bounty-295:
"Hi there - I recently launched LabDAO (https://www.labdao.com/) where we try to open up access to scientific tooling. I would love to integrate with gitcoin bounties, but mainnet gas costs are a real blocker. Do gitcoin bounties support layer2s like gnosis chain or polygon?

I could not see an option for it when I wanted to define my first bounty. Probably I am just missing something"