feat: add testnet support to snapshot resolver

[wa0x6e]

Closes https://github.com/snapshot-labs/workflow/issues/220

This PR adds support for testnet on the snapshot resolver (user-avatar, user-cover, space-cover, space-avatar)

This PR introduces a new address format (NETWORK:ADDRESS), e.g. s:ens.eth or s-tn:0x91FD2c8d24767db4Ece7069AA27832ffaf8590f3, while still be backward compatible with the previous ADDRESS format

How to test

• Mainet space cover: http://localhost:3008/space-cover/test.wa0x6e.eth
• Mainet space cover: http://localhost:3008/space-cover/s:test.wa0x6e.eth
• Testnet space avatar: http://localhost:3008/space/s-tn:diva-testing.eth
• Testnet user avatar: http://localhost:3008/avatar/s-tn:0x3b4e97c176DD99fE73307F123189F53EeF0BC435?resolver=snapshot (should be same as https://snapshot.4everland.link/ipfs/bafybeiaxlwymw6f76enjaf5errbybrfrv5ak67jxyzf7tucdbmn6tsjor4)

Note

This is the first step of a refactoring process.

In a later PR, the space-sx resolver will be merge into this one, and the network prefix will allow targeting the network API, and avoid polling all onchain api on each call